Institute of AstronomyUniversity of Cambridge

Natural Sciences TriposPart III Astrophysics

Project Booklet

2011-2012

Editors: Ian Parry and Judith Moss

Contents

Introduction 3

Supervisor(s) Project Title

Paul Hewett Finding Quasars that are Gravitationally LensingBackground Galaxies

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Gerry Gilmore Evolution of the Milky Way disk: secular vs isolated 6

Mark Wyatt Dusty Resonant Rings 7

Camille Bonvin andAnthony Challinor

Relativistic corrections in the observed distribution ofgalaxies

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Mike Irwin Local Group Satellite Galaxies 12

Ben Davies The nature of the massive young star cluster RSGC3 15

Ben Davies The Formation of Massive Stars 17

George Becker and PaulHewett

Finding hidden galaxies using the most distant QSOs 19

Grant Kennedy Debris disc dynamics in binary systems 21

Mark Gieles, PoulAlexander andWyn Evans

The dynamical evolution of star clusters 22

Jonathan Gair Testing relativity using inspirals of spinning blackholes binaries

24

Martin Haehnelt andGeorge Becker

Probing the Epoch of Reionization with the O1 forest 26

Cathie Clarke Radiation hydrodynamical modelling of X-ray/UVphotoevaporation of discs around young stars

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Cathie Clarke The lack of microlensing events in the globularcluster 47 Tuc and its implications for brown dwarfsin globular clusters

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Simon Hodgkin,Eduardo Gonzalez-Solares and Mike Irwin

An Infrared Hunt for the Nearest Coolest Stars 31

Vasily Belokurov andSergey Koposov

The Galactic Thick Disk in the Infrared 33

Katie Mack andMartin Haehnelt

Modelling the Universe in 3D with 21cm IntensityMapping

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Manda Banerji andRichard McMahon

A Multi-Wavelength Study of the Most DistantGalaxy Clusters

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Chris Tout Eccentricity in common envelope evolution 40

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Ian Parry Fourier Transform Spectrometers for Astronomy 42

Ian Parry Project 1640 44

Craig Mackay Wavefront phase retrieval with curvature sensors 46

Nick Moeckel Breaking planetary mean motion resonances withstellar flybys

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Lukasz Wyrzykowski Transient events as astrophysical tools 48

Mark Gieles,Sverre Aarseth andChristopher Toout

Runaway OB stars from young massive clusters 50

Richard McMahon andManda Banerji

A microwave search for distant accretingsupermassive black holes

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Project Timetable 54

Project Report Format and Content 56

Examiners Criteria for Marking the Project Reportand Oral Presentation

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2

Introduction

This booklet contains descriptions of the individual projects available in Academic Year 2011-2012. Each entry contains a brief description of the background to the project along with asummary of the type of work involved and several references where more information can beobtained. Following the project descriptions, details of the timetable, format of the project write-ups and the criteria to be used in the assessment of the projects are included. The booklet is madeavailable in Sept to allow preliminary consideration of the projects before the start of term.

Ian Parry, Part III Astrophysics Course Coordinator 2011-2012

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Finding Quasars that are Gravitationally Lensing BackgroundGalaxies

Supervisor: Paul Hewett, Office: H19, E-mail: phewett@ast.cam.ac.uk

Background

The Sloan Digital Sky Survey (SDSS) is the largest astronomical survey of the sky at opticalwavelengths and more than 1,500,000 spectra of quasars, galaxies, stars and blank sky areincluded in the Legacy “DR7” release. In addition to the light from the target objectscontributing to each spectrum, the sensitive SDSS spectra are capable of detecting faint emissionfeatures resulting from the gas ionized by massive, hot young stars in galaxies at redshifts up toz=1.3. By identifying such signatures the SDSS has enabled the most successful survey forstrong gravitational lenses by galaxies to be undertaken. Massive, luminous, red galaxies (LRGs)were examined for the tell-tale signs of gravitationally lensed background star-forming galaxies.The analysis of the several hundreds of lenses detected now provides some of the most importantconstraints on the baryonic and dark matter content of massive galaxies.

An outstanding question in galaxy evolution is to better understand the origin of the strongcorrelation between host galaxy mass and the mass of the central black hole. Application of thesame techniques used to identify lenses among the population of LRGs to the population ofrelatively low redshift quasars (or active galactic nuclei) has the potential to define a sample ofquasars lensing galaxies. Subsequent, more detailed, observations of such a sample couldprovide new constraints on the properties of quasar host galaxies and the relationship to thecentral supermassive black holes.

Embarking on a survey for such rare emission line objects hidden in the SDSS spectra providesthe opportunity to undertake a research investigation with a genuinely unknown outcome. Theproblem is more difficult than the LRG-based study and the work could take a number ofdirections depending on how many candidate lenses are discovered. Even if rather fewcandidates are located, quantifying the effectiveness of the survey to constrain the masses of thequasar host galaxies will provide a varied research topic.

Nature of the Project Work

The project will involve working with a large astronomical database, optical magnitudes andspectra.

* The first step will to become familiar with the type of sources that may be identified via asearch for emission lines in the SDSS quasar spectra.

* For a student interested in developing their own computer code, a second step would be to,undertake a search for objects in the SDSS spectroscopic database (a program, in Matlab, IDL,FORTRAN or C for example, will need to be written) to identify emission line sourcesAlternatively, emission line detections can be found using existing computer code.

Subsequent steps will be determined by the nature of, and how many, candidate lenses are foundbut are likely to include:

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* Determine the apparent luminosity of background galaxies identified and estimate the lensinggeometries of the systems and hence the masses of the quasar host galaxies,

* By carefully considering the SDSS spectroscopic selection, assess the statistical significance ofthe lens candidates identified.

References

Courbin, F. et al. 2010, A&A, 516, L12 (first published quasar-galaxy lens)

Bolton, A. et al. 2006, ApJ, 638, 703 (the first SDSS LRG gravitational lens sample)

The SDSS DR7 web page: http://www.sdss.org/dr7/index.html

Figure 1 from Courbin et al. (2010) showing the SDSS spectrum of the quasar J0013+1523,redshift z=0.12, (with spectral features indicated by the vertical black lines) along with thecharacteristic emission line spectrum of a background star-forming galaxy at redshift z=0.64(features indicated by vertical red lines).

5

Evolution of the Milky Way disk: secular vs. isolated

Supervisor: Gerry Gilmore, Office H47, email: gil@ast.cam.ac.uk

Background

Galactic disks are fundamental to structure in the Universe, yet remain impressively notunderstood. One of many aspects of the puzzle is the extent to which disks are long-lived locallystable systems, where stars form at some Galacto-centric radius and stay near that radius for manyGyr; or are stars bounced large distances by dynamical asymmetries and resonances. Essentially,did the Sun form anywhere close to its present location? A key test of integrated Galactic history isstellar chemical abundances – the only property of (most) stars which is invariant from birth. TheGalactic disk seems to have a large-scale radial gradient in mean chemical abundance for youngstars. Hence, if stars have moved large distances, the local abundance distribution at any placeshould have width corresponding to the range of locations which now populate it.

A large survey (RAVE) of which we are members has obtained 500,000 stellar spectra, to addressthis among many questions. This project will involve establishing the external calibration check onthe RAVE chemical abundance calibrations, then determining the shape and width of the localcalibrated abundance distribution. This then provides model limits on radial migration of starsgiven a range of possible radial abundance gradients.

Nature of the Project Work

The project involves accessing the RAVE internal working data system, to access theextremely large data catalogue. This catalogue must be cross-correlated against theavailable spectroscopic calibration libraries (both public, and private libraries underdevelopment for Gaia calibration).

Systematic trends in stellar parameters (T_eff, log g, [Fe/H], [alpha/H],..) must be explored,discovered or limited, and external error estimates on parameter accuracy derived.

The calibrated distribution function(s) can then be quantified. Analysis compared topublished abundance gradient determinations will then be feasible.

Analysis can be performed using any suitable programming language (e.g., IDL, C,Fortran).

References

Freeman, K., Bland-Hawthorn, J. 2002, ARA&A, 40, 487 (review)

Gilmore, G, Wyse, R.F.G., & Kuijken, K 1989 ARA&A 27, 555 (review)

Luck etal arXiv 1106.0182 recent data paper on gradients

Roskar etal arXiv:1101.1202 – review of radial migration and its interest

Ruchti etal arXiv:1105.3691 recent analysis of thick disk abundances, based on RAVE

Siebert etal 2011 AJ 141 187 – the RAVE DR3 data release

Spitoni, E., Matteucci, F. A&A 2011 531 72 recent model of gradients – intro to literature

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Dusty Resonant Rings

Supervisor: Mark Wyatt, Office: H38, Email: wyatt@ast.cam.ac.uk

Background

The inner regions of the Solar System are inhabited by large quantities of micron- to mm-sized dust that is produced by collisions between asteroids and the fragmentation of comets inthe Sun’s debris disk. Poynting-Robertson (P-R) drag forces cause the dust to spiral in towardthe Sun from its source location at a few AU so that the Earth sits directly in the middle ofthis dust cloud. As the dust passes the Earth, some fraction of it becomes trapped in theEarth’s outer mean motion resonances. The resulting over-concentration of dust at 1AU iscalled the Earth’s resonant ring. This ring was first detected by the fact that the geometry ofthe resonances causes the ring to be clumpy, notably with a trailing clump of dust that alwaysfollows the Earth along its orbit (Dermott et al. 1994; see Fig. 1). The ring’s structure hasrecently been observed by Spitzer (Reach 2010), and searches have been made, so farunsuccessfully, for the resonant rings expected to be associated with Mars and Venus.

This project aims to model the formation of resonant ring structures around other stars. It isknown that many nearby stars also host their own debris disk (Wyatt 2008), and althoughmost of those known to date are too dense for P-R drag to be operating (Wyatt 2005),resonant rings may be expected in the fainter disks being discovered more recently. The studyof such structures is particularly important as it may be possible to identify the presence ofEarth-like planets by their effect on the surrounding dust disk much easier than the detectionof the planets themselves.

Nature of the project

Several approaches have been used in the literature to model these resonant rings (e.g.,Kuchner & Stark 2010; Reidemesiter et al. 2011). This project would aim to study thetrapping process itself. Previous work has already been undertaken to determine theprobability of dust grains of different sizes being trapped into the different resonances thatthey may encounter (Mustill & Wyatt 2011; see Fig. 2). This project would determine whathappens to the dust after it is trapped, in particular how long it stays in resonance, how itleaves the resonance, as well as what happens to it afterward (e.g., whether it is subsequentlytrapped into other resonances, or is accreted onto the Earth). Ultimately this informationwould be used to simulate an image of the structure of the resonant rings associated withdifferent planets.

Thus the approach would be partly numerical, using standard N-body integration techniqueslike Mercury to follow the orbital evolution of dust grains past a planet. An analyticalunderstanding of planetary system dynamics would also be useful, and so attendance at thepart III planetary system dynamics course would be beneficial. Some programming would berequired to simulate ring structure models; several pieces of code already exist for thispurpose in IDL, though prior knowledge of IDL is not a pre-requisite.

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References

Dermott S.F., et al. 1994, Nature, 369, 719Kuchner M.J., Stark C.C. 2010, AJ, 140, 1007Mustill A.J., Wyatt M.C. 2011, MNRAS, 413, 554Reach W.T. 2010, Icarus, 209, 848Reidemeister M., et al. 2011, A&A, 527, A57Wyatt M.C. 2005, A&A, 433, 1007Wyatt M.C. 2008, ARAA, 46, 339

Figure 1: Structure of the Earth’s resonant ring. (Left) Face-on view in the frame co-movingwith the Earth’s mean motion (Dermott et al. 1994). The Earth’s orbit is shown with theellipse 1AU to the right of the Sun which is at the centre. (Middle) Close-up of the vicinity ofthe Earth showing the motion of the Spitzer spacecraft through the trailing clump throughoutits 5 year mission. (Right) Polar brightness observed by Spitzer (Reach 2010).

Figure 2: Capture probabilities for dust into different resonances (Mustill & Wyatt 2011).(Left) Capture probabilities into first order resonances as a function of dimensionlessmigration rate (dβ/dt) and dimensionless eccentricities (J). (Right) Application of the left plotto 12 and 120 micron-sized dust passing the j+1:j resonances of the Earth and Mars.

8

Relativistic corrections in the observed distributionof galaxies

Supervisors: Camille Bonvin, Office: K15, E-mail: cbonvin@ast.cam.ac.ukAnthony Challinor, Office: K2, E-mail: a.d.challinor@ast.cam.ac.uk

Background

A key challenge in cosmology is understanding the current accelerated expansion of the Uni-verse. Since the discovery of the acceleration in 1998, cosmologists have designed varioustheories to try and explain it. Two scenarios are mainly explored: the first possibility is to addin the Universe a new exotic component, called dark energy, that has the property to acceleratethe expansion of the Universe; the other possibility is to modify the laws of gravity on largescales. Both types of theory can be designed to give indistinguishable background evolutionsof the Universe, hence measuring the expansion rate does not allow one to distinguish betweenthem. One promising idea to disentangle these models is to look at the formation of struc-ture. The rate at which the large-scale structure in the distribution of galaxies grows is indeedsensitive to the theory of gravity and to the presence of dark energy. Such observations cantherefore, in principle, break the degeneracy between dark energy and modified gravity theories.

Counting the number of galaxies, as a function of direction and redshift, is a powerful wayto measure the growth rate of structure. To use properly this observation it is essential tounderstand precisely what is measured in galaxy surveys. There are indeed observationalcomplications arising from the fact that what we observe does not reflect directly the situationat the source. All the information that we have is inferred from the light that we receive from thegalaxies. Since our Universe is not exactly homogeneous and isotropic, light does not propagateon straight lines but follows perturbed geodesics. As a consequence the number density that weobserve does not, in detail, simply trace the spatial distribution of galaxies. One well-knownexample of such an effect is redshift space distortion: due to peculiar velocities of galaxies,the measured redshift is perturbed with respect to cosmic time. This leads to an apparentsqueezing of galaxy clustering along the direction of observation. Recently, in Bonvin & Durrer(2011) and Challinor & Lewis (2011), it has been shown that, besides redshift space distortion,many other relativistic corrections affect the observed distribution of galaxies. By computingproperly the propagation of light in a perturbed Universe, one finds that the number density ofgalaxies is sensitive to deflection of light (lensing effect), peculiar velocities of galaxies (Dopplereffect) and gravitational potential corrections (Shapiro time delay, Integrated Sachs-Wolfe effectand metric effects at source); see Figure 1. These corrections are too small to be observed incurrent galaxy surveys that are limited to low redshifts and (mostly) small regions of the sky.However, since the corrections grow quickly with angular separation between the galaxies andwith redshift they may be relevant for future large-sky surveys, for example Euclid.

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Nature of the Project Work

The project work will involve a mix of theory and computation and will suit a student confidentin the application of linearised general relativity to observational problems. The project mightinvolve some of the following.

1. Work through the derivation of the relativistic effects and gain familiarity with the nu-merical codes (e.g. CAMB sources) that calculate the observables.

2. Assess how well future galaxy surveys might be able to measure the relativistic correctionsin the observed number density of galaxies. A promising technique to remove the effectsof cosmic variance, and so improve the statistics, is to consider the clustering of differentpopulations of objects which trace the matter distribution with different “bias” (Seljak2009).

3. Consider whether it would be possible to use the number density of galaxies to measureseparately the two metric potentials Φ and Ψ. These two potentials are generally equal ifgravity is described by general relativity, but they may be different in modified theories ofgravity. Measuring the two potentials and comparing them would therefore be extremelyuseful to discriminate between dark energy and modified gravity theories. The idea isto investigate how one could use the various relativistic corrections to separate Φ fromΨ. At small redshift the dominant correction comes from redshift space distortion (seeFigure 1, green and blue lines), which is sensitive to peculiar velocities of galaxies andhence Ψ, whereas at large redshift the dominant correction is due to lensing (magentaline) which is related to the sum of the two potentials integrated along the line of sight.By combining low and high redshift measurements it may be possible to test whetherΨ = Φ.

References

Bonvin C. and Durrer R., 2011. What galaxy surveys really measure. arXiv:1105.5280.Challinor A. and Lewis A., 2011. The linear power spectrum of observed source numbercounts. arXiv:1105.5292.Seljak U., 2009. Extracting Primordial Non-Gaussianity without Cosmic Variance. Phys. Rev.Lett. 102, 021302. arXiv:0807.1770.

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Figure 1: The various contributions to the angular power spectrum (variance of angularfluctuations in the galaxy counts as a function of scale) plotted as a function of multipolel (i.e., inverse angular scale) for four different redshifts: z = 0.1 (top left), z = 0.5 (topright), z = 1 (bottom left) and z = 3 (bottom right). Here a Gaussian observational windowfunction with width ∆z = z/10 is assumed. The different curves are the contributions fromthe density (red), redshift space distortion (green), the correlation of density with redshiftspace distortion (blue), lensing (magenta), Doppler effects (cyan) and gravitational potentials(black). Solid lines denote positive contributions whereas dashed lines denote negativecontributions.

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Local Group Satellite Galaxies

Supervisor: Mike Irwin, Office: APM 1, E-mail: mike@ast.cam.ac.uk

BackgroundThe origin and evolution of galaxies like the Milky Way and M31 and their attendant survivingsatellite systems remain among the key unanswered questions in astrophysics. The galaxieswe see within the Local Group are valuable representatives of the extragalactic populationand have been evolving for the majority of cosmic time. As our nearest neighbors they canbe studied in far more detail than their distant counterparts, and hence provide an excellentperspective on galaxy formation and evolution.

Over the last decade or so the number of known satellite galaxies in the Local Group neigh-bourhood has more than doubled; whilst over the same period more precise measurements oftheir distances and radial velocities have also become available.

The key aims of this project are to revisit several classic (Newtonian) analyses of the mem-bership and properties of Local Group galaxies and to investigate the properties of the M31satellite system, which now numbers around 40 galaxies, and makes up the majority of theLocal Group population.

Nature of the Project WorkThe project will involve the following steps.

a) investigate membership criteria for the Local Group and the two dominant sub-componentsystems of the Milky Way and M31;

b) revisit the timing argument (Kahn & Woltjer 1959) and motion of outer Local Group satel-lites to measure the total mass and age of the system (Einasto & Lynden-Bell 1982);

c) analyse the spatial and velocity distribution of satellites in the M31 for anisotropy and signsof sub-structure, rotation etc...;

d) Can the transverse motion of M31 be accurately constrained from its satellite kinematicse.g. van der Marel Guhathakurta (2008) ?

e) Does modern satellite data provide a good constraint on the solar circular motion e.g. Yahil,Tammann Sandage (1977) ?

f) Estimate the impact of incorporating the modern cosmological ingredients of Dark Matterand Dark Energy.

ReferencesReferences and suggested reading:

Calder, L. & Lahav, O., 2008, AG 49, 13 ”Dark energy: back to Newton ?”Einasto, J.& Lynden-Bell, D., 1982, MNRAS, 199, 67 ”On the mass of the Local Group andthe motion of its barycentre”

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Kahn, F. & Woltjer, L., 1959, ApJ, 130, 705 ”Intergalactic matter and the Galaxy”Nandra, R., et al. 2011, astro-ph 1104.4458 ”The effect of an expanding universe on massiveobjects”Sandage, A., 1986, ApJ, 307, 1 ”The redshift-distance relation: IX. Perturbation of the verynearby velocity field by the mass of the Local Group”van der Marel, R, & Guhathakurta, P., 2008, ApJ, 678, 187 ”M31 Transverse Velocity andLocal Group Mass from Satellite Kinematics”Yahil, A., Tammann, G., Sandage, A., 1977, 217, 903 ”The Local Group - The solar motionrelative to its centroid”

Figure 1: The distances of nearby galaxies from the main mass concentrations around M31and the MW, suggests simple membership criteria for the main sub-groups of galaxies withinthe Local neighbourhood.

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Figure 2: Local Group members and a sample of other nearby galaxies viewed in a Barycen-tric system. The upper and lower tracks are Virial boundaries for purely radial motion andLocal Group mass of 3.0 ×1012 and 4.5 ×1012 M�; whilst the ”knotted” line denotes the limit-ing distance attainable in the notional 13.7 Gyrs since the Big Bang. The position of satelliteswith respect to this latter boundary are quite sensitive to the assumed mass ratio, and hencelocation of the Barycentre, of M31 and the MW.

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The nature of the massive young star cluster RSGC3

Supervisors: Ben Davies, Office: H34, E-mail: bdavies@ast.cam.ac.uk

Background

With the recent major advances in infrared observations, we have finally begun to see through the dust in the Galactic Plane and discover the the Milky Way’s population of massive young star clusters. These clusters have been produced during the Milky Way’s most recent massive starburst events.

One such cluster is RSGC3 (Clark et al. 2009). It is part of a large complex of clusters which appear to be located at the end of the Milky Way’s central bar. While some other clusters in this complex are well studied (Davies et al 2007, 2008), this cluster remains poorly understood. The aim of this project is to use data recently obtained from the Keck telescope in Hawaii to fully explore the nature of this star cluster – to confirm its distance and location in the Galaxy, measure its mass and age, and its evolutionary state.

Nature of the Project Work

• The project uses high resolution spectroscopic data from the Keck telescope – one of the world’s premier facilities. The data will already be largely reduced, but it will be the task of the student to analyse these data. This will be done using the data analysis package IDL. Therefore, familiarity with programming would be advantageous, though experience with IDL specifically is not necessary.

• The student will measure accurate radial velocities of the stars in the cluster, which can then be used to determine the cluster’s mass (e.g. Davies et al. 2011) and location in the Galaxy.

• Effective temperatures for the stars will be measured from their spectra. Then, using the distance, it will be possible to determine luminosities of the stars, and place them on a H-R diagram. From this, the cluster age can be determined.

• Finally, the cluster’s properties will be discussed in the context of the other clusters in the host complex.

References

Clark et al. 2009, A&A 498, 109

Davies et al., 2007, ApJ 671, 781

Davies et al., 2008, ApJ 676, 1016

Davies et al., 2011, MNRAS 411, 1386

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Figure 1: 3-colour image of the young star cluster RSGC3. Red: 8um, showing the hot dust; Green: 3.6um, showing the stars,; Blue: 20cm radio emission, showing the hot ionized gas. High resolution Keck spectroscopy has been obtained for the bright stars in the centre of the cluster, from which it will be possible to determine the cluster’s distance, mass and age.

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The Formation of Massive Stars Supervisors: Ben Davies, Office: H34, E-mail: bdavies@ast.cam.ac.uk

Background One of the major unsolved problems in astrophysics is how stars with masses greater than ~10M

are formed. Part of this problem is observational – massive stars form very quickly, and so are still embedded in their natal clouds when they arrive on the main sequence. To catch a massive star ‘in the act’ of forming, we need to be able to see into the centre of these clouds.

In a recent paper (Davies et al., 2010), a massive star was observed with unprecedented detail, using integral-field spectroscopy – this is a state-of-the-art technique which provides a high spatial resolution observations with a spectrum at every pixel in the image. These data were able to detect: a circumstellar accretion disc; an ionized bi-polar outflow; and a dense ‘torus’ of material surrounding the star (Fig 1). These were among the first ever detections of such structures around a massive protostar, and provide tantalizing evidence that massive stars form in a similar way to low-mass stars like our Sun.

In this project, the student will expand on the above described study by expanding it to a sample of massive protostars from a recently completed survey of young massive stars (Davies et al., 2011), with the aim of providing the best evidence yet for how massive stars form.

Nature of the Project Work • The project involves using data from one of the world’s best facilities: the SINFONI

instrument on the VLT. The first step of the project will be to reduce these data using the instrument-specific software.

• The second part of the project will be to analyse the ‘data-cubes’ – first to make critical tests of the quality of the data-reduction…

• … and then to study the morphology of the spectral features across the field of view. This will include: looking for evidence of circumstellar discs from the CO emission; looking for evidence of bi-polar outflows from the ionized gas emission. These steps will require use of a data analysis package such as IDL. Familiarity with programming will be an advantage, though IDL is fairly straight forward to learn.

• The results from the entire sample will be combined to make quantitative conclusions about how massive stars form.

References

Davies et al., 2010, MNRAS, 402, 1504

Davies et al., 2011, MNRAS, in press (http://arxiv.org/abs/1105.3984)

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Figure 1: The detection of a rotating circumstellar disk around a young massive star. Left: the morphology of the CO absorption in the direction of the star; Right: the kinematics of the CO absorption, showing that it is rotating.

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Finding hidden galaxies using the most distant QSOs Supervisors: George Becker, Office: K19, E-mail: gdb@ast.cam.ac.uk

Paul Hewett, Office: H19, E-mail: phewett@ast.cam.ac.uk Background Neutral gas is the basic ingredient for star formation. Learning where it resides and how it gets converted into stars over cosmic time is therefore fundamental for understanding how galaxies take shape. Quasar absorption line studies have gotten us part of the way there; most of the neutral gas in the early universe can be accounted for in strong absorbers known as “damped Lyman-alpha systems”, or DLAs, and we know much about their metal content and kinematics. What we don't know is what kinds of galaxies host these absorbers, what their star-formation rates are, and how they fit into the broader picture of hierarchical galaxy assembly. The classic problem in identifying DLA host galaxies is that they – potentially – lie directly in front of the bright quasars in whose spectra they were found. This is like trying to find a candle in front of a searchlight, and it has so far prevented us from discovering what, if anything, may lie close to the line of sight. With the discovery of extremely distant (z > 5) quasars, however, we finally have a way around this. The trick is to identify a (relatively) low-redshift DLA in front of a very high-redshift (i.e., distant) quasar. The intergalactic medium then acts as a filter, blocking out the blue light from the quasar. This allows us to search for faint galaxies near the line of sight free from the glare of the quasar. The situation is demonstrated in the figure below. We have performed deep imaging of 11 DLA fields using this trick. These data will allow us to identify the potential DLA galaxies, or at least set excellent limits on the types of galaxies associated with these pockets of neutral gas. Nature of the Project The project will be mainly observational. The imaging data have already been obtained using some of the world's largest optical telescopes (Keck and Magellan). The first step will therefore be to reduce the data, and a knowledge of IDL, IRAF, or other programming language will be very helpful. The student will use the imaging data to search for galaxies that may host the neutral gas. In cases where no galaxy can be unambiguously associated with an absorber (a likely result!), statistical methods will be needed to set limits on the star-formation rate surface density associated with the absorbing gas. Ultimately, the student will interpret the results in the broader context of our knowledge of DLAs and how gas is converted into stars in other galaxies. References DLAs: Wolfe, A., Gawaiser, E., & Prochaska, J., 2003, ApJ, 593, 235 Limits on star-formation in DLA host galaxies: Wolfe, A. & Chen, H.-W., 2006, ApJ, 652, 981

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This technique: O'Meara, J., Chen, H.-W., & Kaplan, D., 2006, ApJ, 642, L9

Figure 1: A schematic overview of our approach to identifying relatively low-redshift DLA galaxies towards high-redshift quasars. The spectrum of a z = 5.8 quasar contains a strong Mg II system, which suggests a high column density of neutral gas (DLA), at z = 2.298. The quasar is at high enough redshift that the intergalactic medium entirely blocks out its light below 6000 angstroms. Light from a z = 2.3 galaxy, however, would fall further to the blue. The vertical dashed line indicated the wavelength where Lyman-alpha emission from the galaxy would be seen. Deep broad-band images in the filters whose curves are shown should allow us to identify even a faint stellar counterpart to the absorption line system.

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Debris disk dynamics in binary systems

Supervisors: Grant Kennedy, Office: H5, E-mail: gkennedy@ast.cam.ac.ukMark Wyatt, Office: H38, E-mail: wyatt@ast.cam.ac.uk

BackgroundLike the Sun, many stars have belts of comets and asteroids, collectively known as debris disks.Unlike the Sun however, many stars exist in binary and multiple systems. Despite the potentialfor more complex dynamics, it appears that being in a binary system does not affect a star’slikelihood of harbouring a debris disk (Trilling et al 2007). This project is inspired by a specificdebris disk observed around the 99 Herculis binary system by the Herschel space telescope.In this system both the stellar pole and debris disk appear misaligned with the binary orbitalplane, which suggests that the system is the result of a stellar encounter when the system wasyoung. Given that the geometry of such encounters is largely random, binary and multiplesystems with debris disks at random orientations relative to their orbital planes should exist.Because disk particles are subject to perturbations over long timescales, these disks will takeon a range of unusual geometries.

Nature of the Project WorkThis project is about the long term evolution of debris disk particles in binary systems andtheir observational characteristics. This work will be done using the Swift n-body code, andcompared with analytic results where possible (attending the part III course on dynamics willbe beneficial). Different binary mass ratios, eccentricities, and particle inclinations will beexplored. In the coplanar circumbinary case, the importance of these parameters on the diskeccentricity, observed as an offset disk (Kalas et al 2005) or pericenter glow effect (Wyatt etal 1999), will be studied. An initial study of stability has been done by Doolin & Blundell(2011), though the student may wish to seek explanations for some of their results. In the caseof an outer binary, the range of disk structures that result from disk particles with a range ofinclinations can be explored.

ReferencesDoolin, S. & Blundell, K. 2011 arXiv:1108.4144Kalas, P., Graham, J. R., & Clampin, M. 2005, Nature, 435, 1067Trilling, D. E. et. al. 2007, ApJ, 658, 1289Verrier, P. E. & Evans, N. W. 2009, MNRAS, 394, 1721Wyatt, M. C. et. al. 1999, ApJ, 527, 918

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The dynamical evolution of star clusters

Supervisors: Mark Gieles, Office: Hoyle 40, E-mail: mgieles@ast.cam.ac.ukPoul Alexander, Office: Kavli 14, E-mail: pera@ast.cam.ac.ukWyn Evans, Office: Hoyle 50, E-mail: nwe@ast.cam.ac.uk

BackgroundGlobular clusters are ancient relics of star formation in the early Universe and they are foundin the haloes of nearly all galaxies (see Fig. 1 for an example of a massive globular cluster inthe Milky Way halo). The chaotic motions of the stars in a cluster can be compared to thoseof molecules in a gas, with the big difference that star clusters never reach thermal equilib-rium because of their negative heat capacity (e.g. Lynden-Bell & Wood 1968). Energy flowscontinuously from the core of the cluster to the outer parts, where energy is transferred byclose encounters between individual stars. This process, called relaxation, drives the evolutionof star clusters, eventually leading to complete dissolution. This makes star clusters a uniquekind of self-gravitating systems.

This project involves the analyses of a large set of direct N -body simulations of star clustersevolving in a tidal field due to the galaxy they are in. These simulations have been done withSverre Aarseth’s N -body code NBODY6, developed here in Cambridge, using Graphic Process-ing Units (GPUs) for accelerated force calculation. The goal of this project is to analyse thevelocity distribution of stars, and the evolution thereof. It is not well understood yet how theexternal tidal field of the galaxy affects the orbits of stars in the cluster. The large amountof simulations we have available will allow us to find a quantitative description of the velocitydistribution as a function of the cluster properties (such as mass and radius) and the tidal field.The results will be an important ingredient of a rapid cluster evolution package, currentlyunder construction (Alexander & Gieles 2011).

Nature of the Project Work• You will analyse N -body simulations of star clusters with 1 024 ≤ N ≤ 65 536. The

data can be analysed using standard programming languages (such as Fortran, IDL orPython).

• The velocity distributions will be compared to models of self-gravitating systems thatinclude the effect of velocity anisotropy (for example the Osipkov-Merritt models).

• Parametrize the results to get a simple description of the evolution of the entire velocitydistribution in terms of properties of the cluster (mainly the number of stars, the radiusof the cluster and the tidal field strength).

• There will be substantial room for expanding on the results in a direction preferred byyou (i.e. doing more N -body simulations, include alternative tidal field descriptions, etc.)

• The aim is to include the results in a publication in MNRAS

22

ReferencesAlexander, P., & Gieles, M., 2011, in prep (copies available on request)Gieles, M., Heggie, D. C., & Zhao, H.-S. 2011, MNRAS, 413, 2509Fukushige, T., & Heggie, D. C. 2000, MNRAS, 318, 753Lynden-Bell, D., & Wood, R. 1968, MNRAS, 138, 495

Figure 1: Example of a globular cluster: the massive (∼ 106 M�) globular cluster 47 Tucanaewith an average density of ∼ 103 M� pc−3 and a central density of ∼ 105 M� pc−3. Closeencounters between stars are important and they determine how the cluster as a wholeevolves.

23

Testing relativity using inspirals of spinning black hole binaries Supervisors: Jonathan Gair, Office: H63, E-mail: jgair@ast.cam.ac.uk Anthony Lasenby, Office: K28, E-mail: a.n.lasenby@mrao.cam.ac.uk

Background

Although optical observations of the orbital decay of compact binary systems has provided strong indirect evidence for the existence of gravitational waves (GWs), they have not yet been directly detected by man-made instruments. This will change in the next few years, as a ground based network of detectors (LIGO, Virgo etc.) will soon be operating in “Advanced” configurations for which regular detections are likely, and at the end of this decade these detectors will be complemented by a miliHertz GW detector in space, LISA. Observations of GWs from the inspiral and merger of binaries comprising two black holes can be used to measure the parameters of the system to accuracies of a fraction of a percent, but can also be used to precisely test the theory of gravity by comparing the observed signals to the predictions of general relativity.

Previous studies of tests of relativity using GWs have mostly looked at specific alternative theories of gravity, such as massive graviton or scalar-tensor theories, but there have been recent attempts to devise generic models that aim to capture the effects of all possible deviations in the theory in a single framework. Such models have been developed for comparable mass non-spinning binary systems (Yunes & Pretorius 2010) and extreme-mass-ratio systems including spin for the more massive body (Gair & Yunes 2011). The only investigation of comparable-mass spinning binary systems to date made use of the non-spinning corrections to the frequency evolution, and included precessional spin affects only in the general relativity part of the waveform model (Huwyler et al. 2011). It is known that spin precessional modulations help to break parameter degeneracies and improve measurement accuracies for general relativistic binaries (Lang & Hughes 2006), and so we might expect to derive more powerful constraints on deviations in the theory of gravity from looking at spinning binaries. The aim of this project is to develop a model for spinning binaries in generic theories of gravity by including corrections in the waveform precessional modulation, and use these models to explore the accuracy with which future GW detectors will be able to test the theory of gravity.

Nature of the Project Work

• The project will involve some analytical work, to include spin-orbit modulations (taken from the extreme-mass-ratio model) and spin-spin modulations (included at lowest order in an approximate way) into the existing model of comparable mass binaries.

• There will also be a computational element to the project, since once the model has been developed, it should be used to estimate the precision with which the parameters characterising the deviations from GR can be constrained by observations with LIGO and LISA. This work will use the Fisher Matrix formalism, and (if time permits) these results can be verified using Markov Chain Monte Carlo techniques.

• The project can be extended to explore the systematic errors that would arise in the measured parameters if deviations from GR were ignored in the models used to analyse the data when they were actually present in the signal.

• A further extension would be to look at how constraints could be improved by combining the results from multiple observations of different binaries (Berti et al. 2011).

24

References

Berti, E, Gair, J R, and Sesana, A, 2011, arxiv:1107.3528

Gair, J R, and Yunes, N, 2011, Phys. Rev. D in press, arxiv:1106.6313

Huwyler, C, Klein, A, and Jetzer, P, 2011, arxiv:1108.1826

Lang, R N, and Hughes, S A, 2006, Phys. Rev. D74, 122001 (arxiv:gr-qc/0608062)

Yunes, N, and Pretorius, F, 2009, Phys. Rev. D80 122003 (arxiv:0909.3328)

ArXiv references may be downloaded from http://xxx.soton.ac.uk/ .

Figure 1: Sample waveform for an extreme-mass-ratio inspiral system described by general relativity (solid red line) and described by a modified-gravity theory which differs by a small (dashed green line) or somewhat larger (dotted blue line) amount from relativity. Reproduced from Gair & Yunes (2011).

25

Probing the Epoch of Reionization with the OI forest Supervisors: Martin Haehnelt, Office: K27, E-mail: haehnelt@ast.cam.ac.uk George Becker, Office: K19, E-mail: gdb@ast.cam.ac.uk

Background The formation of the first stars and black holes led to the heating, reionization and pollution of the inter-galactic medium with metals. Observations of the microwave sky, together with studies of the Lyα forest in QSO absorption spectra have revealed that the reionization of hydrogen occurred in an extended fashion between redshifts z ~15 and z~6 when the Universe was less than 10% of its present age. Traditional tracers of the ionization state and metal enrichment are, however, “running out of steam” at redshifts z>5. Absorption by neutral oxygen (OI) should be an excellent bet to push studies of neutral gas and its enrichment with metals from the first stars/galaxies to higher redshift into the epoch of reionization.

Nature of the Project Work • In a first step the project involves producing artificial QSO absorption spectra which

include absorption by neutral oxygen from cosmological hydro-dynamical simulations of the inter-galactic medium. This can be done using any suitable programming language (e.g., IDL, C, Fortran).

• In a second step the simulated QSO absorption spectra will be compared to observed spectra to investigate how dense regions in the universe became ionized and enriched with metals.

References

Becker G. 2011, ApJ, 735, 93

Oh P. 2002, MNRAS, 363, 1021

Oppenheimer, B. D., Dave, R., & Finlator, K. 2009, MNRAS, 396, 729

Petitjean, P. 1998, Chapter 15 in Les Houches lectures on “Formation and Evolution of Galaxies”,

O. Le Fevre and S. Charlot (eds.), arXiv: astroph/9810418

Figure 1: Hydro-dynamical simulation of the enrichment of the inter-galactic medium with metals by the first galaxies during the epoch of reionization from Oppenheimer et al. (2009).

26

Radiation hydrodynamical modelling of Xray/UVphotoevaporation of discs around young stars.

Supervisor: Cathie Clarke, Office: H10, E-mail: cclarke@ast.cam.ac.uk

Background

Discs of gas and dust surround young stars for several million years, during which planet formationgets under way, and are then dispersed (e.g. Haisch et al 2001). It is now recognised that a majorfactor in dispersing such discs is the effect of energetic radiation from the young star (particularlyXrays) in heating and photoevaporating the disc (Owen et al 2010). Simulations show that thisprocess first erodes a hole in the disc and then the outer disc is stripped away on timescales ofhundreds of thousands of years.

Recently a new effect has been discovered in calculations of photoevaporating discs (termed`thermal sweeping', Owen et al 2011) such that the disc evaporation suddenly accelerates, and theresidual gas is removed on a very short timescale (i.e. hundreds of years) which is of order the localorbital timescale of the disc. The development of this thermal sweeping can be seen as the plume ofgas that starts to roll over the inner rim of the disc in the right hand panel of the top row of theFigure: in the bottom panel one sees the material lifting out of the disc mid-plane and the residualdisc is rapidly dispersed. The implications of such accelerated gas removal for the evolution of thesolid component of the disc (i.e. dust and rocks) have not yet been explored, but this is importantfor determining the properties of residual rocky debris discs and for continued rocky planetformation at large radii.

First, however, it is necessary to make a more thorough investigation of this `thermal sweeping'phenomenon, through dedicated high resolution simulations which improve the treatment of theinteraction between gas at the disc's inner rim and Xray and ultraviolet radiation. This projecttherefore involves the setting up of a one-dimensional grid-based hydrodynamical code of thisflow, paying particular attention to the temperature structure of the irradiated inner disc rim:heating by Xrays and far ultraviolet radiation will be included using prescriptions by Owen et al2010, Richling & Yorke 2000 amd Gorti & Hollenbach 2008. This calculation will help toestablish whether, and for what range of parameters, this rapid gas removal is initiated.

Nature of the Project Work

The project involves the student writing a one-dimensional hydrodynamical code in order tomodel the flow of gas off the inner rim of an irradiated disc and to model heating by Xraysand ultraviolet radiation in a parametrised form. The computational resources required aremodest and the coding can be written in any suitable programming language (e.g., IDL, C,Fortran).

The results will be compared with existing 2D hydrodynamical simulations for the simplest(Xray only) case and then the effect of varying various parameters of the disc and radiationfield will be explored.

27

References

Gorti, U., Hollenbach, D. 2008, ApJ, 613, 424

Haisch, K., Lada, E., Lada, C. 2001, ApJL, 553, L153

Owen, J., Ercolano, B., Clarke, C., Alexander, R., 2010, MNRAS, 401, 1415

Owen, J., Clarke, C., Ercolano, B., 2011. Submitted to MNRAS

Richling, S., Yorke, H., 2000. ApJ, 539, 258

Figure 1: An image of the thermal sweeping phenomenon from the simulations of Owen et al 2011.Note the plume of gas that develops in the right hand panel of the upper row and whose lifting out

of the disc plane leads to the rapid destruction of the disc in the lower panels.

28

The lack of microlensing events in the globular cluster 47 Tucand its implications for brown dwarfs in globular clusters

Supervisor: Cathie Clarke, Office: H10, E-mail: cclarke@ast.cam.ac.uk

Background

Globular clusters are the oldest well studied stellar populations and it is obviously of interest toknow whether the stellar initial mass function (IMF) was the same at the epoch of globular clusterformation (~ 10 Gyr ago) as it is today. Unfortunately, stars in globular clusters can be directlyobserved over a rather small mass range, since more massive objects have already evolved andbrown dwarfs are too old and cool to be directly detectable.

Bonnell et al 2003 however pointed out that the lack of detection of microlensing events in theglobular cluster 47 Tucanae (Gilliland 2000) may provide an indirect constraint on the populationof (invisible) free floating brown dwarfs in globuar clusters. The microlensing searches weredesigned to detect planets orbiting stars in globular clusters and would also be sensitive to browndwarfs in close orbits around stars.

The lack of detections is particularly interesting because, even if brown dwarfs were never formedas close companions, one would expect a population to form by tidal capture, provided that thecluster originally contained a significant population of free floating brown dwarfs. Bonnell et alplaced an upper limit on the population of free floating brown dwarfs that were consistent with thelack of microlensing detections but did not use a detailed model to describe how the brown dwarfsevolve dynamically within the cluster compared with the stars. This (together with the fact thatestimates of the brown dwarf population in nearby star forming regions have been reviseddownwards since 2003: Andersen et al 2008) means that the conclusions of Bonnell et al 2003 needto be revisited using a more realistic dynamical model.

This project will therefore revisit this problem using the models of Kruijssen (2009) which providefits to the results of Nbody simulations of globular clusters. In particular these models include thepreferential evaporation of low mass objects from highly evolved systems like globular clusters. Itis obviously important to include this effect in order to explore how estimates of the expectednumber of brown dwarfs in tidal capture binaries is related to the initial population of brown dwarfsin the cluster. The project therefore involves the adaptation of codes describing the Kruijssen modelto suit the problem at hand and a re-assessment. (based on the latest observational estimates of theincidence of brown dwarfs at the present epoch) of whether the IMF was deficient in brown dwarfsat the epoch of globular cluster formation.

Nature of the Project Work

The project involves modification of an existing code (written in IDL) in order to explorethe link between the initial poplulation of brown dwarfs in 47 Tuc and the number ofmicrolensing events expected.

A literature search of the latest estimates of the incidence of brown dwarfs at present epochsand an assessment of any evidence for a depressed population in 47 Tuc.

29

References

Andersen, M., Meyer, M.,,Greissl, J., Aversa, A., ApJ, 683, L183

Bonnell, I., Clarke, C., Bate, M., McCaughrean, Pringle, J., Zinnecker, H., 2003. MNRAS 343, L53

Gilliland, R. et al 2000. ApJ, , 545 ,L47

Kruijssen, J.M.D., 2009. Astron. & Astrophys., 507, 1409

Figure 1: An image of the globular cluster 47 Tuc where the lack of microlensing events implies alack of brown dwarfs in tidal capture binaries. The project will determine whether this implies thatthat brown dwarf formation was not favoured in 47 Tuc or whether the brown dwarfs could justhave been lost by tidal stripping from the outer cluster.

30

An Infrared Hunt for the Nearest and Coolest Stars

Supervisors: Simon Hodgkin, Office: APM A5, E-mail: sth@ast.cam.ac.ukEduardo Gonzales-Solares, Office: APM A4 E-mail eglez@ast.cam.ac.ukMike Irwin, Office: APM A1, E-mail: mike@ast.cam.ac.uk

Background

The nearest known star to the Sun is still Proxima Centauri, discovered almost a centuryago as a faint proper motion companion to Centauri AB (Innes 1915). In recent years, anumber of all-sky red (SDSS) and infrared (2MASS, Denis) surveys have uncoveredsignificant numbers of extremely cool and nearby stars and brown dwarfs, principallyidentified from their faint magnitudes and unusual colours. It seems that the solarneighbourhod may actually be dominated by L and T dwarfs, stars cooler than spectralclass M. The UKIDSS surveys have found ever fainter and cooler T dwarfs, but have yetto reveal the elusive Y-dwarfs, the next spectral type expected in the sequence, probablydominated by the presence of Ammonia in the atmosphere, and likely to havetemperatures <500 Kelvin. In the summer of 2011, the WISE project (NASA) finallyannounced the discovery of 6 Y-dwarfs. These ultracool brown dwarfs are more similarin many ways to isolated gas giant planets than they are to stars, and enable us tounderstand the least massive products of the star formation process, as well as sheddinglight on the atmospheric chemistry of super-Jupiters.

Nature of the Project Work

We are proposing to search for extremely cool and nearby Y-dwarfs in ongoing infraredsurvey data, using a two-pronged approach. Firstly we will investigate the colour-colourand colour-magnitude space of the newest VISTA (particularly the VISTA HemisphereSurvey, PI:McMahon) and WISE surveys. Secondly we will examine the significantoverlap within and between the multiple epochs of the UKIDSS and VISTA surveys tosearch for very high proper motion stars. Candidate cool stars from this analysis will befed into existing follow-up re-imaging and spectroscopy campaigns from 4-8 metre classtelescopes in the northern and southern hemispheres.

31

References:

Kirkpatrick et al. 2010, ApJS, 190, 100Burningham et al. 2008, MNRAS, 391, 320Lucas, P. et al. 2010, MNRAS, 408, 56VISTA surveys :http://www.eso.org/sci/observing/policies/PublicSurveys/sciencePublicSurveys.html#VISTAUKIDSS : http://www.ukidss.org/CASU (data processing) : http://casu.ast.cam.ac.uk/WISE : http://wise.ssl.berkeley.edu/

Figure 1: Left: Near-infrared spectra of T dwarfs (from Kikpatrick et al. 2010). Right:VISTA public survey coverage as of August 2010.

32

The Galactic Thick Disk in the Infrared

Supervisors: Vasily Belokurov, Office: H20, E-mail: vasily@ast.cam.ac.ukSergey Koposov, Office: H32, E-mail: koposov@ast.cam.ac.uk

BackgroundAs first recognised by Burstein (1979), describing the distribution of stars in spiral galaxiesrequires at least two disk-like components: one, luminous and thin and another, faint andthick. Subsequently, Gilmore and Reid (1983) discovered the thick disk in our own Galaxy.The Milky Way’s thick disk and its extragalactic counterparts turned out to be made of starsthat are typically older and more metal-poor then the main think disk population (Reid &Majewski, 1993, Chiba & Beers 2000, Yoachim & Dalcanton 2008). For three decades theorigin of the thick disks has remained an open question. Three types of formation mechanismshave been suggested: i) heating the kinematically cold thin disk with mergers (Quinn et al1993), molecular clouds (Villumsen, 1985), spiral arms or dark matter substructure (Hayashi& Chiba 2006); ii) star formation at distances high above the mid-plane (Kroupa 2002, Brook2004); iii) satellite accretion and subsequent dispersal (Gilmore et al 2002, Abadi et al 2003,Read et al 2008).

In the Galaxy, to test these formation hypotheses, thin disk, halo and thick disk propertiesincluding age and metallicity distributions are required. It then may come as a shock thatsuch basic parameters of the Galactic thick disk as scale length and scale height are not knownwith any great accuracy. While individual papers quote errors of order of 20% (e.g. Juric etal 2008), the range of possibilities across various studies is much larger (e.g. Table 1 of Siegel2002). Until recently, the degeneracy was mostly driven by the limited area of the surveydata used in the analysis. In the last decade, the scarce pencil beams of the first Galacticsurveys have been superseded by the overwhelmingly large datasets from surveys like SDSScovering tens of thousands of square degrees. While the wide-area optical imaging data havehelped resolve some of the degeneracies, these constraints came mostly in locations unaffectedby intervening dust, away from the disk plane. To get a handle on the thick disk’s scale-length,a large baseline in Galacto-centric radial distance is required and, hence, low latitude regionsin the central Galaxy must be probed.

Nature of the Project WorkIn this project, you will use the infrared imaging data collected with the wide-field cameraon the 4m UKIRT telescope to measure the structural parameters of the thick disk. Becauseinfrared magnitudes are less affected by the dust extinction, the stellar density distribution closeto the Galactic plane can be straightforwardly inferred. You will do this by fitting thick diskdensity laws to star counts in cells of right ascension, declination, stellar colour and magnitude.Figure 1 shows the view of the Milky Way in the infrared, while Figure 2 gives an example ofthe stellar density in colour-magnitude space.

33

ReferencesAbadi et al, 2003, ApJ, 597, 21Brook et al, 2004, ApJ, 612, 894Burstein, 1979, ApJ,234, 829Chiba & Beers, 2000, AJ, 119, 2843de Jong et al, 2010, ApJ, 714, 663Gilmore & Reid, 1983, MNRAS, 202 1025Gilmore et al, 2002, ApJ, 574, 39Hayashi & Chiba, 2006, PASJ, 58, 835Juric et al, 2008, ApJ, 673, 864Kroupa, 2002, MNRAS, 330, 707Quinn et al, 1993, ApJ, 403, 74Read et al, 2008, MNRAS, 389, 1041Reid & Majewski, 1993, ApJ, 409, 635Robin et al, 1996, A&A, 305, 125Siegel et al, 2002, ApJ, 578, 151Villumsen, 1985, ApJ, 290, 75Yoachim & Dalcanton, 2008, ApJ, 683, 707

Figure 1: The 2MASS infrared view of the Galaxy.

34

Figure 2: Density of stars in the direction of Galactic coordinates l=31◦, b=10◦ in J-K, Kcolour-magnitude space.

35

Modelling the Universe in 3D with 21cm Intensity Mapping

Supervisors: Katie Mack, Office: K35, E-mail: mack@ast.cam.ac.uk Martin Haehnelt, Office: K27, E-mail: haehnelt@ast.cam.ac.uk

Background

Mapping the large-scale structure of the universe primarily relies on finding bright tracers of the underlying matter distribution -- quasars, galaxies and galaxy clusters. However, this method requires finding and characterizing huge numbers of objects, and it is only effective at mapping the very highest peaks in the matter distribution. A new technique of mapping the universe by studying the neutral hydrogen in and around galaxies has recently been developed. 21cm intensity mapping relies on the radiation produced in the hyperfine transition of ground-state neutral hydrogen (which has a wavelength of 21cm). By studying the 21cm transition as it occurs at different redshifts, we can make three-dimensional maps of the density of hydrogen in the universe. This is especially useful for studying the universe after the epoch of reionization, when most of the hydrogen in the intergalactic medium (IGM) is ionized, but neutral pockets remain in dense regions.

Observational attempts to employ 21cm intensity mapping are already underway, and they are already producing an exciting new view of the universe. However, to use these observations effectively, we need to understand how they correspond to the physical properties of the IGM -- this requires accurate models of the hydrogen distribution and the observational signal. The primary aim of this project will be the development of a new model of the 3D distribution of hydrogen in the universe. The model will then be used to produce simulated intensity maps (see Figure 1) to predict what might be observed by future radio telescopes.

Nature of the Project Work

• The project will begin with the production of a model of the dark matter mass distribution as a function of redshift. This can be done with analytical approximations such as that developed by Sheth & Tormen (1999) or through employing numerical routines. This segment of the project (and others) will require some programming (e.g., C or Fortran).

• The next stage will be deriving the neutral hydrogen density from the dark matter density in objects, using a prescription developed in previous work by Barnes & Haehnelt (2009) based on the observed hydrogen abundance in damped Lyman-alpha systems (DLAs).

• Once the model for the hydrogen mass function is complete, simulated intensity maps will be produced for redshifts of approximately 1-6 (after the epoch of reionization), given the specifications of radio observatories such as GMRT, ASKAP, and the SKA. These maps can be compared to future observations to better understand the distribution of hydrogen in the post-reionization universe.

References

Peterson, J.B. et al., 2010, Astro2010 Science White Papers, no. 234 [arXiv: 0902.3091]

Loeb, A. & Wyithe, J.S.B. 2008, PRL 100, 161301

Barnes, L.A. & Haehnelt, M.G. 2009, MNRAS 397, 511

36

Sheth, R.K. & Tormen, G. 1999, MNRAS 308, 119

Figure 1: A simulated intensity map of the brightness in the 21cm transition of neutral hydrogen as a function of redshift (Peterson et al. 2010).

37

A Multi-Wavelength Study of the Most Distant GalaxyClusters

Supervisors: Manda Banerji, Office: H31, E-mail: mbanerji@ast.cam.ac.ukRichard McMahon, Office: H18, E-mail: rgm@ast.cam.ac.uk

Background

Clusters of galaxies are the most massive gravitationally bound systems in the Universe and formin the densest regions of the Universe. The most massive clusters contain thousands of galaxies andhave a total mass including dark matter of up to 1015 solar masses. Measurements of theirabundance and evolution can be used to place constraints on cosmological models such as the ratioof dark matter to dark energy at the formation epoch of the clusters.

In the last few years, new multi-wavelength surveys have allowed distant galaxy clusters to beobserved through X-ray emission from hot intra-cluster gas (Mehrtens et al. 2011) as well as theeffect of this hot gas on the Cosmic Microwave Background temperature (the Sunyaev-ZeldovichEffect – High et al. 2010). This has led to the discovery of some of the first fully formed clusters atredshifts around 1 – the main epoch of galaxy formation in the Universe (Figure 1 – Brodwin et al.2010, Foley et al. 2011). Curiously, many of these clusters appear to already be very massive inapparent contradiction to the standard paradigm of galaxy formation whereby galaxies and clustersgrow in mass hierarchically over cosmic time.

The aim of this project is to use new data at near infra-red wavelengths to study these massivegalaxy clusters in more detail in order to understand the mass assembly history of their constituentgalaxies. The near infra-red data is particularly sensitive to old, massive stars in galaxies andtherefore provides a good proxy for the total stellar mass in galaxies.

The project will make use of new data from the VISTA Hemisphere Survey – the largest near infra-red sky survey currently underway, in order to track the growth of stellar mass in galaxy clustersover cosmic time. The results will then be compared to predictions from large cosmologicalsimulations.

Nature of the Project Work

The project is observational in nature and will involve working with large catalogues aswell as images of galaxies and photometric data at multiple wavelengths.

The multi-wavelength photometric data will be fit to library templates of galaxies usingpublicly available fitting codes in order to infer physical parameters such as the stellarmasses, star-formation rates and amount of dust in the cluster galaxies.

The project will study the evolution in stellar mass of cluster galaxies both as a function ofredshift as well as the cluster properties.

Finally these observations will be compared to predictions from galaxy formation models.

References

Brodwin, M. et al., 2010, ApJ, 721, 90

Foley, R. J. et al., 2011, ApJ, 731, 86

High, F. W. et al., 2010, ApJ, 723, 1736

Mehrtens, N. et al., 2011, MNRAS, Submitted (arXiv:1106.3056)

38

Figure 1: An infra-red image of a high-redshift galaxy cluster at z=1.18 with the contours showingthe Sunyaev-Zeldovich signal in the millimeter. Spectroscopically confirmed cluster members are

marked using circles. (Foley et al., 2011).

39

Eccentricity in Common Envelope Evolution

Supervisor: Christopher Tout, Office: H61, E-mail: cat@ast.cam.ac.uk

BackgroundCommon envelope evolution is an important process in the evolution of binary stars thatexplains how a once wide binary system can shrink its semi-major axis by a factor of up to onethousand or so. For instance cataclysmic variables have a white dwarf accreting from, typicallya low-mass main-sequence companion. The periods of these systems are of the order of a fewhours and their separations a few solar radii. However the white dwarf could only have formedas the core of a red giant of some hundred to a thousand solar radii in size. Thus the systemmust once have been much wider in order to accommodate such a giant. Our understandingof the physics of common envelope evolution is rudimentary. A red giant that fills its Rochelobe while more massive than its companion suffers dynamically unstable mass transfer andleaves a system in which the giant’s envelope engulfs the relatively dense companion and itsown degenerate core. These two then spiral together by some unknown frictional force. Theenergy lost from their orbit goes into blowing off the envelope. The result can be the progenitorof a cataclysmic variable.

It is usually assumed that the process of common envelope evolution leaves the system in acircular orbit but this depends on how angular momentum is transferred along with energy.There is mounting evidence, from the eccentricities of barium stars to models of binary starevolution in dense clusters, that many systems must emerge from their common envelope phasein eccentric orbits. The aim of this project is to investigate what properties of the interactioncould lead to eccentricity growth.

Nature of the Project WorkThis project is theoretical and involves both analytic and numerical modelling. You will firstmake a simple model of the red giant envelope as an n = 3/2 polytrope. In the simplest casethe companion will be modelled as a point mass orbiting in the gravitational field of the giant’sdegenerate core, thus sweeping out it’s path through the polytropic envelope. Two types of dragwill be tested. First the standard Bondi-Hoyle accretion drag and secondly a drag proportionalto the differential rotation between the core and envelope. By integrating the loss of bothangular momentum and energy from the orbit its semi-major axis and eccentricity evolutioncan be modelled. The goal will be to find what kind of interaction leads to eccentricity growand what to circularisation. Various extensions to the project can be envisaged by relaxingsome of the many assumptions or by considering different mechanism for the friction.

ReferencesWebbink R. E., 2008, in Milone E. F., Leahy D. A., Hobill D. W., eds, Short-Period BinaryStars: Observations, Analyses and Results, Springer, Berlin, p. 233

40

Meyer F., Meyer-Hofmeister E., 1979, A&A, 78, 167

Bonaic Marinovic A. A., Glebbeek E., Pols O. R., 2008, A&A, 480, 797

Figure 1: Common-envelope evolution. After dynamical mass transfer from a giant, a com-mon envelope enshrouds the relatively dense companion and the core of the original giant.These two spiral together as their orbital energy is transferred to the envelope until eitherthe entire envelope is lost or they coalesce. In the former case a close white-dwarf andmain-sequence binary is left, initially as the core of a planetary nebula. Magnetic braking orgravitational radiation may shrink the orbit and create a cataclysmic variable. Coalescenceresults in a rapidly rotating giant which will very quickly spin down by magnetic braking.

41

Fourier Transform Spectrometers for Astronomy

Supervisor: Ian Parry, Office: H57, E-mail: irp@ast.cam.ac.uk

Background

Many of the spectrometers found in modern research labs are Fourier Transform Spectrometer(FTSs). Spectrometers of this type are currently hardly ever used by astronomers who prefer to usediffraction grating-based spectrometers (DGSs). However, for the next generation of extremelylarge telescopes (ELTs - see figure 1) and for the planned spectroscopic mega-surveys there is apressing need for change.

FTSs offer very important advantages over DGSs such as compactness, value-for-money and thecapability to measure spectra with very high spectral resolution. So why don't astronomers likethem? One problem with conventional FTSs is that to work properly the light source beinganalysed must be constant throughout the total exposure time. This can be readily achieved in a labenvironment but it is very hard to achieve using a ground-based telescope, mostly because ofatmospheric variations. Last year, a part-III student (Prashin Jethwa) successfully devised a schemeto allow FTSs to work accurately even with a variable input light source. Another problem withFTSs is the complex way in which experimental errors propagate through from the measuredinterferogram to the final spectrum. This makes astronomers wary of the results they give.

So what's wrong with DGSs and why do we need to change anything? Firstly, Scaling up DGSs towork on the next generation of 20m - 40m diameter ELTs is a huge problem. Spectral resolution isinversely proportional to the telescope's primary mirror diameter. It's also proportional to thephysical size of the grating used in a DGS (the grating is a special mirror that's used to split thelight up in to its component colours). So to keep the spectral resolution constant we have to have agrating which is 4 times wider if we go from the largest present-day telescope (10m) to the biggestplanned future telescope (~40m). The whole DGS has to scale linearly by 4x so it ends up being 64times heavier – an enormous and expensive monster! (See figure 2). Secondly, severalspectroscopic mega-surveys are currently being planned (e.g. 4MOST and BigBoss) where 10million spectra will be obtained in a 5 year project. To do this requires the use of manyspectrographs simultaneously and again FTSs potentially offer significant advantages.

The aim of this project is to make detailed and accurate computer simulations of how well an FTSwill perform compared to a DGS for two specific observational projects of high astrophysicalimportance.

Nature of the Project

a) Learn how both types of spectrometer work. Basically a DGS creates a real spectrum (intensityas a function of wavelength) which is recorded directly by a detector. On the other hand, an FTS isan interferometer which can be scanned to produce an interferogram. Scanning means varying thepath difference of the FTS with time. The data taken is therefore a measure of intensity versus pathdifference. The desired spectrum is then obtained by taking the Fourier transform of theinterferogram.

b) Make a computer simulation of the interferogram that would be obtained for an astronomicalobject. For example, take an accurately measured spectrum of a star from an archive and take the

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inverse Fourier transform of it to make the interferogram. As a check that everything is ok take theFourier transform of the interferogram to see that the original spectrum is recovered properly.

c) Repeat b) but now add realistic noise sources (photon noise, detector noise, etc.) and scanningstrategies. For each time sample, generate a full interferogram and record the one data point in itthat would have been recorded at that moment in time. Then construct a full simulation of anobserved interferogram from a series of such points. Take the Fourier transform of this to see howthe derived spectrum compares to the original (real) one.

d) Make an equally realistic simulation of observations made with a DGS.

e) Repeat c) and d) for two very specific observational scenarios.

1) An imaging FTS on an ELT designed to look for the earliest galaxies (at redshifts ofabout 8) to help us understand the early universe and the epoch of reionisation.

2) High spectral resolution fibre-optic observations of large samples of stars to do "chemicaltagging" to understand the assembly history of our Milky Way Galaxy.

References1. http://www.eso.org/sci/facilities/eelt/2. http://www.eso.org/sci/facilities/eelt/instrumentation/index.html3. Bernier et al., 2008, "Technical improvements and performances of SpIOMM: an imaging

Fourier transform spectrometer for astronomy", Proc SPIE vol 7014.[http://arxiv.org/ftp/arxiv/papers/0806/0806.0776.pdf]

4. Jethwa, W, 2011, "Fourier Transform Spectrometers for Extremely Large Telescopes", part-IIIastrophysics project report.

Figure 1: The proposed European Extremely Large Telescope (E-ELT).

Figure 2: The EAGLE spectrograph for the E-ELT. This figure comes from a design study for amulti-object spectrograph for the E-ELT.

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Project 1640: A high contrast imaging system on the5-m Hale Telescope

Supervisor: Ian Parry, Office: H57, E-mail: irp@ast.cam.ac.uk

Background

Observational exoplanet science has grown enormously over the last 15 years. However, most ofthe observational evidence obtained so far is indirect, i.e. we don't directly detect photons from theplanet but detect its effect on its parent star (e.g. radial velocity measurements or transits). Project1640 (or P1640 for short) is an imaging camera designed to be able to see an extremely faintcompanion object (e.g. a planet) right next to a very bright star. It uses several technical tricks toachieve this (see figure 1).

1. An adaptive optics system to correct for atmospheric turbulence and sharpen up the image2. A coronagraph to block the light from the bright star3. A calibration system to correct for optical aberrations in the coronagraph4. A special camera (integral field unit - IFU) that simultaneously takes 27 images at different

wavelengths in the wavelength range 1.1 - 1.7 microns

The reason P1640 takes 27 images is because the speckle pattern from the parent star scales withwavelength so the speckles (which look like planets or faint stars) appear to move with wavelengthwhereas real planets and stars do not. Hence it is possible to distinguish between real objects andspeckles.

P1640 has been in use for the last 3 years on the 5m Hale telescope at Mount Palomar. In thisperiod it was being refined and developed and it was also used to do some science. From Oct 2011it will be used to do a major 100 night survey from Palomar to search for faint companions tobright stars.

Nature of the Project

The raw data that comes out of P1640 is complicated and it needs sophisticated processing toproduce the optimal high-contrast images. Figure 2 shows how the raw 2D data is converted into adatacube.

Data processing code already exists (written in IDL). The aim of this project is to further developand perfect this code and to work with old and new data to search for faint companions. Whiledoing this project the student will learn about the new field of high contrast imaging for thedetection of exoplanets.

References

1. Hinkley, S, "High Contrast Imaging with Coronagraphy and Integral Field Spectroscopy",Ph.D. thesis, Columbia University, 2009.

2. Hunt,S, "The Direct Dectection of Low Mass Stellar Companions", Ph.D. thesis, CambridgeUniversity, in prep. 2011.

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3. Hinkley, S, et al, "A new high contrast imaging program at Palomar Observatory", arXiv, 1012-0008, PASP, 2011.

4. Zimmerman, et al, "Parallactic Motion for Companion Discovery: An M-Dwarf OrbitingAlcor", arXiv:0912.1597, Ap.J., 2010

Figure 1: Schematic of how P1640 works.

Figure 2: P1640 data processing. In the raw 2D image there is a short spectrum for each pixel in thefield of view (see the 3rd image from the left). Each spectrum can be broken down into 27 intensity

values one for each wavelength from 1.1 to 1.7 microns (the vertical direction). The data finallyends up as a 3D "datacube", i.e. a stack of 27 images with two spatial dimensions and a third

dimension of wavelength.

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Wavefront phase retrieval with curvature sensors

Supervisor: Craig Mackay: O25, E-mail: cdm@ast.cam.ac.uk

Background

The highest resolution images ever taken combined Lucky Imaging with low order adaptive opticwavefront sensors. A new instrument is under development that will use a new approach towavefront sensing combining high-speed photon counting detectors with low order very highsensitivity curvature wavefront sensors. In a curvature sensor intensity images are taken in fourplanes, two on either side of the pupil plane at different distances from it. These images are takencontinuously at high-speed and the data analysed to generate the most reliable wavefrontinformation possible. The derivation of the wavefront errors must be completed in real time (andtherefore perhaps 50-100 times per second) and must be as reliable as possible within the limitsof relatively low signal-to-noise data.

There are a number of reduction strategies for fitting a wavefront to the sort of data we will have.The oldest one, the Gerchberg-Saxton algorithm has significant problems with speed ofconvergence and its demands of computer processing power. Other algorithms appear toconverge much more rapidly and are likely to be much better suited to working with photon shotnoise limited data.

Nature of the Project

The project will require understanding of the optical arrangement of the curvature sensorand the photon counting detector systems as well as understanding the way that the lightis propagated through the system.

A review of algorithms used for wavefront phase recovery will then be needed, looking atspeed of convergence and computational simplicity.

The project will require a significant amount of computer programming work, mostlyusing C/C++ although it may be convenient to prototype some of the procedures inMatlab, for example.

The project output should be an assessment of the different strategies, their advantagesand disadvantages in the context of the lucky imaging application.

If time allows, the implementation of the best of these algorithms in Graphics ProcessingUnits, GPU's could be investigated (we are currently using Nvidia Tesla cards for otheraspects of lucky image data reduction).

References

General background on Lucky Imaging: The Lucky Imaging Website at:www.luckyimaging.com

N.M. Law, C.D. Mackay, R.G. Dekany, M. Ireland, J. P. Lloyd, A. M. Moore, J.G.Robertson, P. Tuthill, H. Woodruff, (2009) "Getting Lucky with Adaptive Optics: FastAO Image Selection the Visible with a Large Telescope", ApJ 692 p.924-930.

O. Guyon, et al, (2008), "Improving the Sensitivity of Astronomical Curvature WavefrontSensor Using Dual Stroke Curvature", PASP, 120, 655.

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Breaking planetary mean motion resonances with stellarflybys

Supervisors: Nickolas Moeckel, Office: H53, E-mail: moeckel@ast.cam.ac.ukMark Wyatt, Office: H38, E-mail: wyatt@ast.cam.ac.uk

Background

As planets form out of the protoplanetary disc surrounding their host star, their interactions with thedisc can lead to changes in their orbital properties. One possible outcome of these interactions isconvergent migration, where planets on potentially well-separated orbits are driven closer together.When two planets are separated such that their orbital frequencies are ratios of integers, e.g. 2:1 or3:2, they can enter a mean motion resonance, a dynamical state where certain combinations of theorbital parameters are bounded about some constant value. Convergent migration can drive planetsinto this resonant state (e.g. Lee & Peale 2002). At the moment, there is some tension between thenumber of these resonant systems seen in reality and the number predicted from simulations ofplanetary dynamics in the presence of viscous discs (e.g. Matsumura et al. 2010; Moeckel andArmitage 2011)

There are a few possible mechanisms to break resonances, for example turbulence in the disc atearly times (Adams et al. 2008). A relatively unexplored resonance disruption mechanism isexternal perturbations to the planetary system from other stars. Depending on how deeply intoresonance a system is, a weak perturbation could be sufficient to disrupt the system, and theseperturbations can easily be provided by other stars in the host star's cluster. Quantifying theimportance of this mechanism relies on an understanding of how large a perturbation is needed tobreak resonances; the goal of this project is to determine the necessary encounter parameters tobreak up a variety of resonant configurations.

Nature of the Project Work

We will use existing theoretical frameworks to attempt to analytically estimate the magnitude ofperturbations necessary to break resonances, but much of the work is expected to be numerical. Asuite of n-body experiments will be performed to explore the effect of a stellar passage on anexisting resonant planetary system. The bulk of the project will thus be computer intensive,performing these simulations and analysing their output.

References

Adams, F. C., Laughlin, G. and Bloch, A. M., 2008, ApJ, 683, 1117

Lee, M. H. and Peale, S. J., 2002, ApJ, 567, 569

Matsumura, S., Thommes, E. W., chatterjee, S. and Rasio, F. 2010, ApJ, 714, 194

Moeckel, N. and Armitage, P. J., 2011, MNRAS in press. arXiv:1108.5382

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Transients events as astrophysical tools

Supervisors: Lukasz Wyrzykowski, Office: H5, E-mail: wyrzykow@ast.cam.ac.ukPaul Hewett, Office: H19, E-mail: phewett@ast.cam.ac.uk

BackgroundTransient events such as supernovae, novae or microlensing events, play an increasingly im-portant role in modern astrophysics. They usually last from minutes to weeks, therefore theirtemporal nature implies the need for rapid detection, classification and extensive follow-upstudy during the short period when they are visible, as they usually never repeat. So far, thou-sands of supernovae were detected in real-time and many of the type Ia events were employedin precise determinations of the distance scale of the Universe and studies of Dark Energy.Novae are a broader class of cosmic explosions, but also are being used in measuring distancesto nearby galaxies. Thousands of gravitational microlensing events found in our Galaxy andbeyond are helping understand the nature of Dark Matter and provide a unique way of findinginvisible planets.

Recent discoveries from the Palomar Transient Factory (PTF, Quimby et al. 2011) show thatwe still do not know everything about supernovae. PTF reported that 4 of the transients theyhave detected form a new subclass of super-luminous supernovae. Because of their brightnessthese objects, if found in larger quantities, could be potentially used in measuring distances togalaxies further away than it is possible now with classical supernovae. This, in turn, couldlead to a better understanding of the nature of Dark Energy and the expansion and fate of theUniverse.

Nature of the Project WorkIn this project we propose to perform a search for candidates of super-luminous supernovaein the time-domain data collected by the SDSS collaboration in the region of the sky calledStripe 82. The catalogue produced by Bramich et al. (2008) contains several millions of objectsfound using a difference imaging method, hence it should contain objects of temporal nature.The super-luminous supernovae are expected to be visible in the short-wavelength u-band. Theproject will require devising an automated transient detection algorithm, which may also detectother kinds of transients, including novae and microlensing events. The statistics of transientsfindings will help in preparations for ESA’s space mission Gaia, which will be launched in 2013and will be detecting transient events from the entire sky.

The working computing environment will be, preferably, Linux/MacOSX, with tools like IDL,TopCat and ds9 astronomical image viewer. In the course of the project, programming in alanguage like IDL or Matlab will be necessary. The data will be accessed via an SQL database,but examples of the approach to database mining will be provided.

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References• Bramich, D. et al. 2008, MNRAS 386, 887, http://arxiv.org/abs/0801.4894

• Quimby, R. et al. 2011, Nature 474, 487, http://arxiv.org/abs/0910.0059

Figure 1: Set of super-luminous supernovae found in the PTF project exhibiting excessat ultraviolet (UV) wavelengths. Upper panel shows the detection, lower shows the pre-transient image.

Figure 2: Candidate UV transient found in the Stripe 82 data. The object is visible in theu-band (upper left) but not in the g-band (upper right). Bottom images show previous (left)and next (right) epoch available for this part of the sky.

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Runaway OB stars from young massive clusters

Supervisors: Mark Gieles, Office: Hoyle 40, E-mail: mgieles@ast.cam.ac.ukSverre Aarseth, Office: Hoyle 11, E-mail: sja@ast.cam.ac.ukChristopher Tout, Office: Hoyle 61, E-mail: cat@ast.cam.ac.uk

BackgroundYoung massive clusters (YMCs) are found in nearly all star-forming environments and are par-ticularly abundant in starburst galaxies (see Portegies Zwart, McMillan & Gieles 2010 for areview). Fig. 1 shows a high resolution image from the Hubble Space Telescope of R136 in theTarantula nebula in the Large Magellanic Cloud, a famous and well studied example becauseof its proximity. YMCs are abundant in massive (M ∼> 10 M�) stars and are characterisedby a high density. Spectroscopic studies of massive star populations indicate that the binaryfraction among these stars is at least 50%, with many mass ratios close to unity and shortperiods (about 10 − 100 days). These properties are particularly interesting from a dynamicalpoint of view, because the periods are so short that these systems have binding energies highenough to survive interactions with other single and binary stars. Such interactions do takeplace in the cores of dense YMCs and lead to energetic escapers of massive stars and sometimesof the binary itself because of the recoil.

In this project you will make direct N -body simulations with the Cambridge NBODY6 code(Aarseth 2003) of YMCs with realistic numbers of stars (N ≈ few ×105) and realistic propertiesof the primordial binary population. The code also treats variety of astrophysical processessuch as mass loss of stars and collisions. It has only recently become feasible to do this type ofsimulation because of Graphic Processing Units (GPUs) used for accelerated force calculation.The goal is to see whether the presence of a massive star population that is often found aroundYMCs (see Fig. 1) can be explained by ejections from the dense core of the cluster.

Nature of the Project Work• You will simulate the first few Myrs of the evolution of a YMC such as R136 with a

variety of initial conditions for the cluster and the binary population, based on recentobservational results (Evans et al. 2011). The simulations can be analysed with standardprogramming languages (such as Fortran, IDL or Python).

• You will look at the properties of the escaping stars (and binary stars!), such as theirvelocities and masses, and also the type of interaction that created the ejection (binary-binary, binary-single, three-body, etc.).

• An additional topic of interest is the feedback of these interactions on the cluster as awhole, in particular the energy budget, because energy must be conserved.

• The results will be compared to recent results of the VLT Flames Tarantula Survey, aspectroscopic survey of about 1000 massive stars in and around R136 (Evans et al. 2011).

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ReferencesAarseth, S. J., 2003, Gravitational N-Body Simulations. Cambridge: Cambridge Univ. Press.430 pp.Evans, C. J., et al. 2011, A&A, 530, A108Portegies Zwart, S. F., McMillan, S. L. W. & Gieles, M., 2010, ARA&A, 48, 431

Figure 1: Example of the nearest Young Massive Cluster, R136 in the Tarantula nebula(30 Doradus) in the Large Magellanic Cloud. It is a few Myrs old, has a mass of about105 M� and an exceptionally high central density of about 105 M� pc−3. Interactions betweenbinary and single stars are dominating the dynamics of the central region and can eject alarge number of massive stars from the cluster with high velocities, in a short time-scale,comparable to the age of the cluster.

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A Microwave Search for Distant Accreting SupermassiveBlack Holes

Supervisors: Richard McMahon, Office: H18, E-mail: rgm@ast.cam.ac.ukManda Banerji, Office: H31, E-mail: mbanerji@ast.cam.ac.uk

Background

The South Pole Telescope (SPT) is a new 10m telescope that is sensitive to microwave radiation inthe mm wavelength range. The South Pole is an ideal observing site since it is cold and dry andhence the atmosphere is relatively transparent to microwave radiation which is absorbed bymoisture.

The SPT can detect redshifted thermal dust and synchrotron emission from relativistic electronsfrom distant star-forming galaxies and quasars. The dust has a temperature of 30-50K and isbelieved to both form in and be heated in regions of intense star formation where stars are formingat a unsustainable rate of 1000 to 10,000 Solar masses per year within the galaxies that surroundaccreting supermassive black holes that are observed as quasars. The aim of this project is todetermine the nature of around 100 new sources of microwave radiation that have been detected bythe South Pole Telescope at a wavelength of 1mm and 2mm.

The microwave sources are expected to be either star-forming galaxies with redshifts in the range0.1 to 1.0 or active galaxies and quasars harbouring supermassive black holes fuelled by merginggas rich star-forming galaxies in the redshift range 0.5 to 3.0. New infra-red images from theVISTA telescope in Chile and the WISE satellite will be used to determine the nature of the SPTdetected microwave sources and to determine whether the emission is coming from stars in our owngalaxy, distant galaxies or quasars.

The project will involve analyzing catalogues of data and visually inspecting the infra-red imagesfrom the VISTA telescope and WISE satellite. The work will involve using existing computerprograms as well simple statistical techniques to analyze the data.

Nature of the Project Work

The work is a mixture of observational and computational and will involve working withlarge catalogues as well as images at a range of different wavelengths.

The project will involve analyzing catalogues of infra-red data from the VISTA telescopeand WISE satellite and microwave data from the SPT and other radio surveys and satellitesand also visually inspecting the infra-red images from the VISTA telescope and WISEsatellite.

The work will involve both using and modifying existing computer programs as well simplestatistical techniques to analyze the data.

The photometric data from different wavelengths will be used to determine which and howmany of the SPT sources are high redshift dusty quasars.

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References

South Pole Telescope website: http://pole.uchicago.edu/

WISE satellite website: http://wise.ssl.berkeley.edu/

Dust-free quasars in the early Universe, Jiang et al, 2010, Nature 464, 380.The far-infrared-submillimetre spectral energy distribution of high-redshift quasars, Priddey, R.McMahon, R.G., 2001, MNRAS, 324P, 17.The discovery of the first luminous z ~ 6 quasar in the UKIDSS Large Area Survey, Venemans, B.,McMahon R.G. et al, 2007, MNRAS, 376, 76.Extragalactic Millimeter-wave Sources in South Pole Telescope Survey Data: Source Counts,Catalog, and Statistics for an 87 Square-degree Field, Viera, J.D. et al, 2010, ApJ, 719, 763.

Figure 1. Artists impression of a quasarcontaining an accreting black hole andsurrounded by a dusty torus or a disrupteddusty gas rich merging galaxy.

Figure 2. The 10m South Pole Telecope

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Project Timetable

Michaelmas Term An orientation course (5 lectures) covering unix, the Institute of Astronomy Science Cluster, LaTex (text-processing facility) and information resources available on-line commences on the first Thursday of Michaelmas Full Term (6 October 2011).

Choice of up to five projects, in rank order, should be handed to Judith Moss by 4:30pm on the second Friday of Michaelmas Full Term (14 October 2011). Students who do not supply rank-ordered choices by the deadline will be allocated a project by the Part III Course Coordinator.

Notification of approval of project choice will be made by e-mail no later than the third Tuesday of Michaelmas Full Term (18 October 2011).

The equivalent of 3 formal Supervisions will be offered by the Project Supervisor.

An interim progress report, length no more than 1000 words, bearing the signature of the supervisor and responsible University Teaching Officer (UTO), must be handed to Judith Moss no later than the last day of Michaelmas Full Term (2 December 2011). The report should be produced using LaTex, or an equivalent text-processing package, and may contain material that can be incorporated in the final project report. The report must indicate the progress so far and give a clear indication of the aims and how results will be achieved. This is particularly important where the results of the project depend on data that has yet to be analysed. The progress reports do not constitute part of the formal assessment but are regarded as an essential part of the monitoring procedure.

Lent Term The equivalent of 3 formal Supervisions will be offered by the Project Supervisor.

A practice oral presentation, consisting of a 20 minute talk followed by up to 10 minutes of questions, to an audience of Part III Astrophysics students, Project Supervisors and the Part III Course Coordinator will be given on the last Thursday of Lent Term (15 March 2012). A final timetable for the presentations will be provided by e-mail during the previous week. The presentation is not formally assessed but offers the opportunity to become familiar with the format of the presentation, to be assessed by the Part III Examiners, in the Easter Term. The Project Supervisor’s attendance at the informal presentation and subsequent feedback constitutes the fourth, and final, Supervision of the Lent Term.

Easter Term A draft of the final project report, generated using LaTex or an equivalent text-processing package, should be handed to the Project Supervisor no later than 19 April 2012. An eighth Supervision, to discuss the draft report, should take place no later than the first Tuesday of Easter Full Term (24 April 2012).

Two copies of the final project report must be handed in person to Judith Moss no later than 4:30pm on the second Tuesday of Easter Full Term (1 May 2012). It is essential that your report is identified only by your name. Your University Examination Number must NOT appear anywhere in the report. Late submissions must be submitted via your College Tutor with an accompanying letter of explanation from the Tutor.

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A formal, assessed, oral presentation to Part III Astrophysics Examiners, will take place on the second Thursday or Friday of Easter Full Term (3 or 4 May 2012). A final timetable for the presentations will be provided via e-mail during the previous week. The presentation should consist of a 20 minute description of the project using PowerPoint on a laptop computer. The presentation will be followed by up to 10 minutes of questions. The Examiners will allocate approximately 15% of the total marks for the project on the basis of the presentation.

NST Part III Astrophysics Examiners meeting takes place on Tuesday, 19 June 2012.

Project reports may be collected from Judith Moss after 9:00am on Wednesday 20 June 2012.

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Project Report Format and Content The report should read as a self-contained document, presented in the style of a scientific research report or paper in a scientific journal. The main sections of the report will describe the work undertaken, the results obtained and an assessment of their significance. An Abstract, Introduction, Conclusions and References will also be included. Supporting Figures and Tables should be used both as an aid in presenting data and results and also to enhance the clarity of the submission. In some circumstances an appendix containing more extensive tabular material/results may be included.

Reports should consist of a text of no more than 8000 words, not counting Figures, Tables, Captions, References, Equations and any Appendices. The submission must be produced with LaTex, or another text-processing package, with computer generated figures. You must include a declaration that the text does not exceed 8,000 words. Projects found to exceed this limit will be returned for shortening and a penalty will apply for late submission.

The submission should be logically structured, clear and complete, while remaining concise. The reader should be able to understand the context in which the investigation was undertaken, the main features of the project, the results, and how they relate to the advancement of the subject. In addition to the descriptive material, questions a report would be expected to address include, “why were particular approaches adopted?” − back of the envelope calculations will often be helpful and relevant − “what has been learnt?” and “what information/work would have helped to learn more?”

It is a fundamental tenet of scientific research that due acknowledgment is given to the work and ideas of others that form the basis of, or are incorporated in, a research presentation. You must always acknowledge the source of an idea or material, including figures, you use with a specific reference. Plagiarism, including the use of another individual’s ideas, data or text is regarded as an extremely serious disciplinary offence by the University. It is a requirement that the project investigation and the project report are both the work of the candidate alone and any form of collaboration is not allowed.

Two copies of the final project report must be handed in person to Judith Moss no later than 4:30pm on the second Tuesday of Easter Full Term (1 May 2012). Late submissions must be submitted via your College Tutor with an accompanying letter of explanation from the Tutor. Your University Examination Number must NOT appear anywhere in the report or on the cover sheet (see next paragraph).

Each report must be accompanied by a cover sheet that should bear (1) the title of the project, (2) your name, (3) your college, (4) your home address and (5) a signed declaration that reads:

I declare that this project report represents work undertaken as part of the NST Part III Astrophysics Examination. It is the result of my own work and, includes nothing which was performed in collaboration. No part of the report has been submitted for any degree, diploma or any other qualification at any other university and it does not exceed 8000 words, excluding Figures, Tables, Captions, References, Equations and any Appendices. I also declare that an electronic file containing this work has been emailed on this date Signed................. Date: ………………………

If you are in any doubt as to whether you can sign such a declaration you should consult the Part III Course Coordinator before submitting your report.

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In the event that your project report is not collected after the Examinations the report will be sent to the address provided on the cover sheet.

Examiners Criteria for Marking the Project Report and Oral Presentation The project element of the NST Part III Astrophysics course constitutes one third of the course (equivalent to the marks assigned to two 24-lecture Mathematics Part III lecture courses). Approximately 15% of the marks for the project will be assigned on the basis of the assessed oral presentation that takes place in the Easter Term. The balance of the marks will be assigned on the basis of the written project report. The Examiners will award marks under three broad headings, i) scientific understanding, ii) quality of the research, iii) presentational and communication skills.

The format and timetable for submission form part of the Examination process. In their assessment of the project, the Examiners will take account of any breaches of the guidelines, including exceeding the word limit and late submission of the report.

Oral Presentation The Examiners assessment will take into account the following:

• Visual Material: including relevance, clarity, attractiveness

• Oral Presentation: including overall structure, clarity, time keeping

• Response to Questions: including grasp of subject material, precision of answers

Project Report The Examiners will assess the report under the following headings:

• Overall structure and clarity of the report

• Planning, organisation and prosecution of the research

• Understanding of the physics and the general scientific content

• Technical proficiency

• Analytical and Interpretational skills

• Significance of the results

Special Examination Arrangements Any student who believes there are circumstances that require special treatment by the examiners must ensure that this information is communicated to the Course Secretary by their College at the earliest opportunity and well before the project presentations, see http://www.admin.cam.ac.uk/offices/exams/students/special_12.pdf.

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