Structure and Evolution of the Milky Way

Ken FreemanResearch School of Astronomy & Astrophysics

RED GIANTS AS PROBES OF THE STRUCTURE AND EVOLUTION OF THE MILKY WAYRome November 2010

Issues:

Building the thin disk : its exponential radial structure, the role of mergersStar formation history, chemical evolution, continuing gas accretion Evolutionary processes: disk heating, radial mixing The outer disk: chemical gradient and chemical properties

The thin disk: formation and evolution

Many of the basic observational constraints are still uncertain:

• The star formation history of the disk• How does the metallicity distribution in the disk evolve with time• How do the stellar velocity dispersions evolve with time

Measuring stellar ages is still a major problem

The galactic disk shows an abundance gradient(eg galactic cepheids: Luck et al 2006) ....

Carney & Yong 2005

+ cepheids, other symbols are open clusters in the Galaxy. Clusters have ages 1-5 Gyr, cepheids are younger

The abundance gradient and [/Fe]-gradient in the disk has flattened with time,tending towards solar values. For R > 12 kpc, abundance gradient disappears

Metallicity gradient in outer regions of M31disk also bottoms out, as in the Milky Way

Worthey et al 2004

M31

The age-metallicity relation in the solar neighborhood is still uncertain

Rocha-Pintoet al 2006

Edvardsson et al 1993Nordstrom et al 2004Valenti & Fisher 2005

Estimating agesfor field stars is

difficult

(Reid et al 07)

The large scatter in [Fe/H] at all ages was part of the reason to invoke largescale radial mixing : bring stars from inner and outer Galaxy into the solar neighborhood

Our preliminary age-metallicity relation for about 400 nearby subgiants. Ages derived from isochrones in the log g - Te plane via high resolution spectra Gently declining A-M relation with rms scatter of only 0.15 dex in [M/H] (scatter includes the [M/H] error of ~ 0.10). Less need for radial mixing.

Wylie de Boer et al 2010

(Gyr)

log age

subgiants

What is the observed form of the heating with time ?The observational situation is not yet clear ...

• One view is that stellar velocity dispersion ~ t 0.2-0.5

eg Wielen 1977, Dehnen & Binney 1998, Binney et al 2000 …

stellar age

velocitydispersion

(km/s)total

W = 0.4total

Wielen 1977

W is in the vertical (z) direction

(McCormick dwarfs, CaII emission ages)

Wielen age-velocity

relation (AVR)

Edvardsson et al (1993) measured accurate individual velocities and ages for ~ 200 subgiants near the sun.

Their data indicate heating for the first ~ 2 Gyr, with no significant subsequent heating. Disk heating in the solar neighborhood appears to saturate after 2 Gyr, when z ~ 20 km/s.Soubiran et al (2008) measured sample of clump giants, and agree.

Difficulty of measuring stellar ages is reason for the different views. Accurate ages from asteroseismology would be very welcome. Accurate ages and distances for giants would allow us to measure the AMR and AVR out to several kpc from the sun.

• Another view is that heating occurs for the first ~ 2 Gyr, then saturates because stars are mostly away from the Galactic plane

Freeman 1991; Edvardsson et al 1993; Quillen & Garnett 2000

Velocity dispersionsof nearby F stars

old disk

thickdisk

Disk heating saturates at 2-3 Gyr

appears atage ~ 10 Gyr

The thick disk and halo of NGC 891 (Mouhcine et al 2010): thick disk has scale height ~ 1.4 kpc and scalelength 4.8 kpc, much as in our Galaxy.

Most spirals (including our Galaxy) have a second thicker disk component

The Formation of the Thick Disk

Thick disk

Our Galaxy has a significant thick disk

• its scaleheight is about 1000 pc, compared to 300 pc for the thin disk

• its surface brightness is about 10% of the thin disk’s. • it rotates almost as rapidly as the thin disk• its stars are older than 10 Gyr, and are

• significantly more metal poor than the thin disk : mostly (-0.5 > [Fe/H] > -1.0)

• alpha-enriched so its star formation was rapid

From its kinematics and chemical properties, the thick disk appears to be a discrete component, distinct from the thin disk

Freeman 1991; Edvardsson et al 1993; Quillen & Garnett 2000

Velocity dispersionsof nearby F stars

old disk

Thick disk is kinematically distinct

thickdisk

appears atage ~ 10 Gyr

[( + Eu)/H] vs [Fe/H] for thin and thick disks near the sun

Navarro et al (2010), Furhmann (2008) The thick disk is chemically distinct

Veltz et al (2008) analysed the kinematics of stars near the Galactic poles in terms of components of different W. The figure shows the weights of the components: the kinematically distinct thin and thick disks and the halo are evident.

thin disk 225 pc

thick disk 1048 pc

halo

Ivezic et al 2008

see the thick disk up to z ~ 4 kpc: [Fe/H] between -0.5 and -1.0

-2.0 -1.5 -1.0 -0.5 0.0 [Fe/H]

current opinion is that the thick disk itself shows no vertical abundance gradient (eg Gilmore et al 1995)

The old thick disk is a very significant component for studying Galaxy formation, because it presents a kinematically and chemically recognizable relic of the early Galaxy.

Secular heating is unlikely to affect its dynamics significantly, because its stars spend most of their time away from the Galactic plane.

Yoachim & Dalcanton 2006

Baryonic mass ratio: thick disk/thin disk

• Most disk galaxies have thick disks • The fraction of baryons in the thick disk is typically small (~ 10-15%) in large galaxies like the MW but rises to ~ 50% in smaller disk systems

How do thick disks form ?

• a normal part of early disk settling : energetic early star forming events, eg gas-rich merger (Samland et al 2003, Brook et al 2004)

• accretion debris (Abadi et al 2003, Walker et al 1996). The accreted galaxies that built up the thick disk of the Galaxy would need to be more massive than the SMC to get the right [Fe/H] abundance (~ - 0.7) The possible discovery of a counter-rotating thick disk (Yoachim & Dalcanton 2008) would favor this mechanism.

• heating of the early thin disk by disruption of massive clusters (Kroupa 2002). The internal energy of the clusters is enough to thicken the disk

Clump cluster galaxy at z = 1.6(Bournand et al 2008)

Much recent work on the significance of these high-z clump structures as origin of metal-richglobular clusters (Shapiro et al 2010); origin of thick disk and bulges via merging of clumps and heating by clumps (Bournaud et al 2009)

Clumps form by gravitational instability, generate thick disks with uniform scale height ratherthan the flared thick disks generated by minor mergers. (Recall diamond shape of the thick disk of NGC 891)

• early thin disk, heated by accretion events - eg the Cen accretion event (Bekki & KF 2003): Thin disk formation begins early, at z = 2 to 3. Partly disrupted during active merger epoch which heats it into thick disk observed now, The rest of the gas then gradually settles to form the present thin disk

• thick disk is generated by radial mixing of more energetic stars from the inner early disk (eg Schönrich & Binney 2009)

How to test between these possibilities for thick disk formation ?

Sales et al (2009) looked at the expected orbital eccentricity distribution for thick disk stars in different formation scenarios. Their four scenarios are:

• a gas-rich merger (Brook et al 2004, 2005). The thick disk stars are born in-situ• accretion (Abadi 2003). The thick disk stars come in from outside• heating of the early thin disk by accretion of a massive satellite• radial migration (stars on more energetic orbits migrate out from the inner galaxy to form a thick disk at larger radii where the potential gradient is weaker (Schönrich & Binney 2009)

Sales et al 2009

Distribution of orbital eccentricity of thick disk stars predicted by the different formation scenarios.

Wilson et al 2010, Ruchti et al 2010: f(e) for thick disk stars from RAVE : may favor gas-rich merger picture ?

Abadiby massive satellite

(gas-rich)

Firm control of selection effects is needed in separation of thin and

thick disk stars

• Thick disks are very common.

• In our Galaxy, the thick disk is old, and kinematically and chemically distinct from the thin disk. What does it represent in the galaxy formation process ?

• The orbital eccentricity distribution will provide some guidance.

• Chemical tagging will show if the thick disk formed as a small number of very large aggregates, or if it has a significant contribution from accreted galaxies. This is one of the goals for the HERMES survey.

Thick disk summary

The Galactic Stellar Halo

Rotational velocity of nearby stars relative to the sun vs [m/H] (V = -232 km/s corresponds to zero angular momentum)

rapidlyrotatingdisk &

thick disk

slowlyrotating

halo|Zmax| < 2 kpc

Widely believed now that the stellar halo ([Fe/H] < -1) comes mainly fromaccreted debris of small satellites - cf Searle & Zinn 1978

• Is there a halo component that formed dissipationally early in theGalactic formation process ? eg ELS, Samland & Gerhard 2003

ELS 1986 Halo- building accretions are still happening now - eg Sgr dwarf, NGC 5907

Satellite metallicity distributions not like the metallicity distribution in the halo (Venn 08) - but maybe were more alike long ago.

Fainter satellites are more metal-poor and consistent with the MW halo in their [alpha/Fe] behaviour

NGC 5907: debris of a small accreted galaxyOur Galaxy has a similar structure from the disrupting Sgr dwarf

APOD

Carollo et al (2010 ) identified a two-component halo plus thick disk in sample of 17,000 SDSS stars, mostly with [Fe/H] < -0.5

Describe kinematics well with these three components: <V> [Fe/H]Thick disk 182 51 -0.7Inner halo 7 95 -1.6Outer halo -80 180 -2.2 (retrograde)

From comparison with simulations, Zolotov et al (2009) argue that the inner halo has a partly dissipational origin while the outer halo is made up from debris of faint metal-poor accreted satellites.

Hartwick (1987) : metal-poor RR Lyrae stars show a two-component halo: a flattened inner component and a spherical outer component.

• Is there a halo component that formed dissipationally early in the Galactic formation process ?

Nissen & Schuster (2010): 78 halo stars - see high and low [alpha/Fe] groups. Abundances [Fe/H] > -1.6

The high-alpha stars could be ancient in-situ stars, maybe heated by satellite encounters. The low-alpha stars may be accreted from dwarf galaxies.Note different V-distribution of red and blue points.

Low [/Fe] stars are in mostly retrograde orbits

How much of halo comes from accreted structures ?

Ibata et al (2009) ACS study of halo of NGC 891 (nearby, like MW, but not Local Group) shows lumpy metallicity distribution, indicating that its halo is made up largely of accreted structures which have not yet mixed away.

(cf simulations of stellar halos by Font et al 2008, Gilbert et al 09, Cooper et al 2009)

APOD

Summary for the Galactic stellar halo:

• stellar halo is made up mainly of debris of small accreted galaxies, although there may be an inner component which formed dissipatively

The Galactic bar/bulge

The boxy appearance of the bulge is typical of galactic bars seen edge-on. Where do these bar/bulges come from ? They are very common. About 2/3 of spiral galaxies show some kind of central bar structure in the infra-red.

The bars come naturally from instabilities of the disk. A rotating disk is often unstable to forming a flat bar structure at its center.

This flat bar in turn is often unstable to vertical buckling which generates the boxy appearance.

This kind of bar/bulge is not generated by mergers

The maximum vertical extent of boxy/peanut bulges occurs near radius of vertical and horizontal Lindblad resonances ie where b = - /2 = - z/2(both and z depend on the amplitude of the oscillation)Stars in this zone oscillate on orbits which support the peanut shape.

Orbits supporting the peanut

Edge-on

End onIn-plane

Compare the structure and kinematics of the galactic bulge withN-body simulations of disks that have generated a boxy bar/bulge through bar-buckling instability of the disk (eg Athanassoula). Do the simulations match the properties of the Galactic bar/bulge (eg exponential stucture, cylindrical rotation) ?

How to test whether the bulge formed through the bar-buckling instability of the inner disk ?

The kinematics ofthe model are as

observed forboxy bulges:

cylindrical rotation

b = 0.5°b = 9.5°

The stars of the bulge are old and enhanced in -elements rapid star formation history

If the bar formed from the disk, then are the bulge stars and adjacent disk stars chemically similar ? Not clear yet

Here the data for the bulge stars and thick disk stars come from different sources

Fulbright et al 2007

[/Fe] higher for thick disk than for thin disk: higher still for bulge

bulge

thick disk

thin disk

Meléndez et al 2008

Differential analysis of O-abundance in bulge, thick disk and thin disk stars. The thick disk is O-enhanced relative to thin disk as usual, but the bulge and thick disk look very similar in this study.

In the bar-buckling instability scenario, the bulge structure is probably younger than the bulge stars, which were originally part of the inner disk

The bar-forming and bar-buckling process takes 2-3 Gyr to act after the disk settles

The alpha-enrichment of the bulge and thick disk comes from the rapid chemical evolution which took place in the inner disk before the instability acted

The stars of the bulge and adjacent disk should have similar ages in this scenario. Accurate asteroseismology ages for giants of the bulge and inner disk would be a very useful test of the scenario

If the bulge comes from disk instabilities, then the stars in the bulge were once part of the inner disk: its stars are older than the bulge structure

We are doing a survey of about 28,000 clump giants in the bulge and the adjacent disk, to measure the chemical properties of stars (Fe, Mg, Ca, Ti, Al, O) in the bulge and adjacent disk: are they similar, as we would expect if the bar/bulge grew out of the disk ? We use the AAOmega fiber spectrometer on the AAT, to acquire medium-resolution spectra of about 350 stars at a time : R ~ 12,000

Melissa Ness

Where are the first stars now ? Diemand et al 2005, Moore et al 2006, Brook et al 2007 …

The metal-free (pop III) stars formed until z ~ 4 in chemically isolated sub- halos far away from largest progenitor. If its stars survive, they are spread through the Galactic halo. If they are not found, then their lifetimes are less than a Hubble time truncated IMF

The oldest stars form in the early rare density peaks that lay near the highest density peak of the final system. They are not necessarily the most metal-poor stars in the Galaxy. Now they lie in the central bulge region of the Galaxy.

Accurate asteroseismology ages for metal-poor stars in the inner Galaxy would provide a great way to tell if they are the oldest stars or just stars of the inner Galactic halo. Needs ~10% precision in age.

Brook et al 2007

Distributions of the first stars and the metal-free stars

Bulge rotation for metal rich and metal poor stars

Ness et al 2010

• Is there a small classical merger-generated bulge component, in addition to the boxy/peanut bar/bulge which probably formed from the disk ?

• See a slowly rotating metal-poor component of the bulge. How do we identify the first stars from among the metal-poor stars in the bulge region ?

We seek signatures or fossils from the epoch of Galaxy

assembly, to give us insight about the processes that took place as the Galaxy formed.

The goals of galactic archaeology

A major goal is to identify observationally how important mergers and accretion events were

in building up the Galactic disk and the bulge.

CDM predicts a high level of merger activity which conflictswith many observed properties of disk galaxies.

Try to find groups of stars, now dispersed, that were associated at birth either • because they were born together in a single Galactic star-forming event, or • because they came from a common accreted galaxy.

Aim to reconstruct the star-forming aggregates and accreted galaxies that built up the disk, bulge and halo of the Galaxy

Some of these dispersed aggregates can be still recognised kinematically as stellar moving groups.

For others, the dynamical information was lost through heating and mixing processes, but they are still recognizable

by their chemical signatures (chemical tagging).

The galactic disk shows kinematical substructure in the solar neighborhood: groups of stars moving together, usually called moving stellar groups (Kapteyn, Eggen)

• Some are associated with dynamical resonances (eg Hercules group): don't expect to see chemical homogeneity or age homogeneity (eg Antoja et al 2008, Famaey et al 2008)

• Some are debris of star-forming aggregates in the disk (eg HR1614 group and Wolf 630 group). They are chemically homogeneous; such groups could be useful for reconstructing the history of the galactic disk.

• Others may be debris of infalling objects, as seen in CDM simulations: eg Abadi et al 2003

Stellar substructures in the disk

Look at the HR1614 group (age ~ 2 Gyr, [Fe/H] = +0.2) which appears to be a relic of a dispersed star forming event. Its stars are scattered all around us.

This group has not lost its dynamical identity despite its age.

De Silva et al (2007) measured accurate differential abundances for many elements in HR1614 stars, and found a very small spread in abundances. This is encouraging for recovering dispersed star forming events by chemical tagging

The HR 1614 group is probably the dispersed relic of an old star forming event. V

U

Chemical studies of the old disk stars in the Galaxy can help to identify disk stars which came in from outside in disrupting satellites, and alsothose that are the debris of dispersed star-forming aggregates like the HR 1614 group (Freeman & Bland-Hawthorn 2002)

The chemical properties of surviving satellites (the dwarf spheroidal galaxies) vary from satellite to satellite, and are different in detail from the more homogeneous overall properties of the disk stars.

We can think of a chemical space of abundances of elements O, Na, Mg, Al, Ca, Mn, Fe, Cu, Sr, Ba, Eu for example. The dimensionality of this space is between about 7 and 9. Most disk stars inhabit a sub-region of this space. Stars which came in from satellites may be different enough to stand out from the rest of the disk stars.

With this chemical tagging approach, we may also be able to detect or put observational limits on

the satellite accretion history of the galactic disk

Chemical studies of the old disk stars in the Galaxy can help to identify disk stars that are the debris of common dispersed star-forming aggregates.Chemical tagging will work if

• stars form in large aggregates - believed to be true• aggregates are chemically homogenous• aggregates have unique chemical signatures defined by several elements which do not vary in lockstep from one aggregate to another. Need sufficient spread in abundances from aggregate to aggregate so that chemical signatures can be distinguished with accuracy achievable (~ 0.05 dex differentially)

Testing the last two conditions were the goals of de Silva's work on open clusters : they appear to be true.

See de Silva et al (2009) for more on chemical tagging

Chemical tagging is not just assigning stars chemicallyto a particular population (thin disk, thick disk, halo)

Chemical tagging is intended to assign stars chemicallyto substructure which is no longer detectable kinematically

We are planning a large chemical tagging survey of about a million stars, using the new HERMES multi-object

spectrometer on the AAT.

The goal is to reconstruct the dispersed star-forming aggregates that built up the disk, thick disk and halo within

about 5 kpc of the sun

HERMES is a new high resolution multi-object spectrometer on the AAT

The four wavelength bands are chosen to detect lines of elements needed

for chemical tagging

spectral resolution 28,000(high resolution option 50,000)400 fibres over square degrees4 VPH gratings ~ 1000 ÅFirst light ~2012 on AAT

Strong synergy with Gaia: accurate parallaxes (~ 1% errors) and proper motions

Galactic Archaeology with HERMES

400 fibers in deg2 matches the stellar density at V ~ 14at intermediate Galactic latitudes

4-m telescope, resolution 28,000, expect SNR = 100 per resolution element at V = 14 in 60 minutes

We are planning a large survey reaching to V = 14

Old disk dwarfs are seen out to distances of about 1 kpcDisk giants ……………………………………… 6Halo giants ……………………………………… 15

Fractional contribution from galactic components

Dwarfs Giants

Thin disk 0.58 0.20 Thick disk 0.10 0.07 Halo 0.02 0.03

About 9% of the thick disk stars and about 14% of the thin disk stars

pass through our 1 kpc dwarf horizon

Assume that all of their formation aggregates are azimuthally mixed right around the Galaxy, so all of their formation sites are represented

within our horizon

• 20 thick disk dwarfs from each of ~ 4,500 star formation sites• 10 thin disk dwarfs from each of ~ 35,000 star formation sites

* A smaller survey means less stars from a similar number of sites

Simulations (JBH & KCF 2004) show that a complete random sample of 1.2 x 106 stars

with V < 14 would allow detection of about

HERMES and GAIA

GAIA will provide precision astrometry for HERMES sample

For V = 14, = 10 as, = 10 as yr -1 : this is GAIA at its best

(1% distance errors for dwarfs at 1 kpc, 5 km s -1 transverse velocity errors for giants at 6 kpc)

accurate transverse velocities for all stars in the HERMES sample, and accurate distances for most of the survey stars therefore accurate color-(absolute magnitude) diagram for most of the survey stars: independent check that chemically tagged groups have common age.

GAIA is a major element ofa HERMES survey

The data products include:

• [Fe/H], [/Fe] and [X/Fe] for vast samples of stars from each Galactic component: thin and thick disks from Rg = 2 to 15 kpc halo out to Rg = 20 kpc

• HERMES + Gaia data will give the distribution of stars in [position, velocity, chemical] space for a million stars, and isochrone ages for about 200,000 stars with V < 14

What can the HERMES survey contribute to asteroseismology ?

The main goal of the survey is unravelling the star formation history of the thin and thick disk and halo via chemical tagging.

Chemical tagging in the inner Galactic disk

(expect ~ 200,000 survey giantsin inner region of Galaxy)

The old (> 1 Gyr) surviving open clusters

are all in the outer Galaxy, beyond a radius of 8 kpc.

Young clusters are seen in the inner Galaxy but do not survive the tidal field and the GMCs.

Expect many broken open and globular clusters in the inner disk : good for chemical tagging recovery using giants, and good for testing radial mixing theory. The Na/O anomaly is unique to globular clusters, and may help to identify the debris of disrupted globular clusters.

Radial mixing: transient spiral arm interactions can move stars from one near- circular orbit into another (Sellwood & Binney 2002; Minchev & Famaey 2010).How important is this effect in real galaxies ?

Simulation of disk formation from cooling gas in a dark halo potential (Roskar et al 2008)

• Can we detect ~ 35,000 different disk sites usingchemical tagging techniques ?

Yes: we would need ~ 7 independent chemical element groups, each with 5 measurable abundance levels, to get enough independent cells (57) in chemical abundance space.

• Are there 7 independent elements or element groups ?

Yes: light elements (Na, Al) Mg other alpha-elements (O, Si, Ca, Ti) Fe and Fe-peak elements light s-process elements (Y, Zr) heavy s-process elements (Ba, La) r-process (Eu)

A survey of 1.2 x 106 stars to V = 14with HERMES would take 3000 pointings

Bright time program

~ 9 fields per night for ~ 330 clear nights:5 year program.

• Each galaxy has had a different evolutionary track

• The position of the knee forms a sequence following SFH-timescales (and somewhat the galaxy total luminosity)

• s- process (AGB product) very efficient in galaxies with strong SFR at younger ages (<5Gyrs): Fnx > LMC > Sgr > Scl

• r/s-process elements can be used as another clock (or even 2 clocks: r/s transition knee, and start of rise in s )

• AGB lifetimes + s-process yields are metallicity-dependent (seeds)

• Abundance pattern in the metal-poor stars everywhere undistinguishable ? Seems to be the case for stars in the exended low-metallicity populations.

SNII +SNIa

rise in s-process

LMC Pompeia, Hill et al. 2008Sgr Sbordone et al. 2007Fornax Letarte PhD 2007Sculptor Hill et al. 2008 in prep

+ Geisler et al. 2005Carina Koch et al. 2008

+ Shetrone et al. 2003Milky-Way Venn et al. 2004

Abundance ratios reflect differentstar formation histories

Venn 2008

Hyades Cr 261 HR 1614

De Silva et al 2009

De Silva et al 2009

De Silva et al 2009

There is still much disagreement about stellar age estimates

Edvardsson et al (1993) agesfor subgiants

Nordstrom et al (2004) agesStars mostly near main sequence[Fe/H] and Te from Strömgren photometry

against isochrone ages from Valenti & Fischer (2005)

Measuring accurate stellar ages is difficult(Reid et al 2007)

Data reduction and analysis:

AAO provides basic reduction: extraction, wavelength calibration, scattered light removal,

sky subtraction

Science team provides abundance analysis pipeline, based on

MOOG (C. Sneden),

HERMES wavelength

bands