IAU General Assembly 2009Symposium

Daniela Carollo Macquarie University Research Centre in Astronomy, Astrophysics & Astrophotonics

Department of Physics & Astronomy, Macquarie University

Aboriginal ArtAboriginal ArtMilky Way DreamingMilky Way DreamingG. PossumG. Possum

The Current View of the Halo System of the The Current View of the Halo System of the Milky WayMilky Way

The Galactic Stellar HaloThe Galactic Stellar Halo: Locus of Ancient StarsLocus of Ancient Stars

Their motion has an imprints of the

dynamical history of the Galaxy

Their chemical compositionchemical composition encode information about the

environment environment in which they formed during the early

history of the Galaxy

Two basic information

Fossil Record of the first generation of stars that formed shortly after the Big-Bang

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The halo system

• Smooth components: Inner and Outer Halo

• Substructures

• Overdensities

• Globular Clusters

Inner Halo:

Dominant at R < 10-15 kpc Highly eccentric orbits Slightly Prograde Metallicity peak at [Fe/H] = -1.6

Outer Halo:

Dominant at R > 15-20 kpc More uniform distribution of eccentricity Highly retrograde orbits Metallicity peak around [Fe/H] = -2.2

Smooth Halo ComponentsSmooth Halo Components

D.D. Carollo et al., Nature 2007, Vol. 450, 1020-1025Carollo et al., Nature 2007, Vol. 450, 1020-1025

Halo Halo Halos Halos

Stellar Fractions, FIN and FOUT, as a function of Zmax

The value ZThe value Zmaxmax ~ 15-20 kpc ~ 15-20 kpc

is the inversion point! is the inversion point!

Full [Fe/H] RangeFull [Fe/H] RangeExtracting the Fractions of Each Galactic ComponentExtracting the Fractions of Each Galactic Component

Carollo et al. 2010, ApJ, 712, 692

Inner and Outer Halo Velocity EllipsoidsInner and Outer Halo Velocity Ellipsoids

Inner Halo : Ellipsoid dominated by the radial motionInner Halo : Ellipsoid dominated by the radial motion Outer Halo: Ellipsoid less dominated by the radial motionOuter Halo: Ellipsoid less dominated by the radial motion

Carollo et al. 2010, ApJ, 712, 692

Power-Law Density Profiles for Inner/Outer HaloPower-Law Density Profiles for Inner/Outer Halo

29.079.120.012.3 ; RR outin

Also:

Axial Ratio for the Inner Halo : c/a ~ 0.6 (flattened distribution)Axial Ratio for the Outer Halo : c/a ~ 0.9 (nearly spherical distribution)

Carollo et al. 2010, ApJ, 712, 692

Outer halo population exhibit a much shallower spatial density profile than the Inner halo

Why is the Presence of a Dual Halo so Important?Why is the Presence of a Dual Halo so Important?

It is not just that it is double…

Members of the two components exhibit:

Different Spatial DistributionsDifferent KinematicsDifferent Chemical Composition (MDFs)

Distinct Astrophysical Origin Crucial to Understand the Formation and Subsequent Evolution of the Milky Way

Inner HaloInner Halo: Dissipative radial merger of few massive sub-Galactic fragments formed at early stage, mainly in situ stars

Outer HaloOuter Halo: Dissipationless chaotic merging of small sub-systems within a pre-existing Dark Matter Halo, mainly accreted stars

In the context of the CDM model:

Inner and Outer Halo in Equatorial Stripe 82Inner and Outer Halo in Equatorial Stripe 82(An et al. , submitted)(An et al. , submitted)

Observed photometric MDF is inconsistent with a single [Fe/H] population.

Distinct Halo Populations in the Solar NeighborhoodDistinct Halo Populations in the Solar Neighborhood Nissen & Schuster, 2010Nissen & Schuster, 2010

High [High [/Fe]/Fe] Mainly Prograde OrbitsPrograde Orbits Low [Low [/Fe]/Fe] Mainly Retrograde OrbitsRetrograde Orbits

High- pop.: dissipative component of the Galaxy that experienced a rapid chemical evolution.Low- pop.: accreted from dwarf galaxies that had lower star formation rates.

Evidence of Inner/Outer Halo from SEGUE Vertical Photometry Stripes

(de Jong et al. 2010)

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• Metallicity shift as a function of the Galactocentric distance: [Fe/H] ~ -1.6 at R < 15 kpc ; [Fe/H] ~ -2.2 at R > 15-20 kpc

• Mean stellar mass density exhibit two halo populations with inversion points in the predominance at 15-20 kpc

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Disk/Halos Structure from SEGUE Stellar PhotometryDisk/Halos Structure from SEGUE Stellar Photometry (De Jong et al. 2010)(De Jong et al. 2010)

Left Panel

Black dots: total stellar mass densityRed line: density for the thick-disk-like populationBlue line: density for the inner- halo-like populationGreen line: density for the outer- halo-like population

Right Panel

Fractional contribution of the individual template populations

Inner-outer halo inversion point between 15-20 kpc.In agreement with Carollo et al. (2007-2010).

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BHB and RR-Lyrae towards the Anticenter: Transition BHB and RR-Lyrae towards the Anticenter: Transition between Inner and Outer Halobetween Inner and Outer Halo

Kinman et al., 2012Kinman et al., 2012

Stars become highly retrograde as the Galactocentric distance increases.

Retrograde orbits dominate for Galactocentric distances > 12.5 kpc

Transition between inner and outer halo

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The Dual Halo in the High Resolution Simulations The Dual Halo in the High Resolution Simulations Font et al. 2011

General feature of stellar spheroids of simulated disk galaxies with Milky Way mass:

Metallicity shift in the halo as a function of radius

NB: Metallicities in these simulations are rather arbitrary at present, the important information is that DIFFERENCES are seen as a function of distance

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The Dual Halo in the High Resolution Simulations The Dual Halo in the High Resolution Simulations McCarthy et al., 2012

General feature of stellar spheroids of simulated disk galaxies with Milky Way mass:

Shear exists in the mean rotational velocity between the in situ and the accreted halo component

• Stars with [Fe/H] < -2.0 and [C/Fe] > +1.0

- At least 20% of ALL stars with [Fe/H] < -2.0 are CEMP

- At least 40% of ALL stars with [Fe/H] < -3.5 are CEMP

- Most of the stars with [Fe/H] < -4.0 are CEMP including the two most iron-poor stars (Hyper Metal Poor; [Fe/H] < -5.0) are extremely carbon rich, [C/Fe] ~ + 4.0

CEMP stars contain useful information on the nature of CEMP stars contain useful information on the nature of nucleosynthesis in the early Galaxy.nucleosynthesis in the early Galaxy.

Where does this carbon come from?

Distinct Chemical Pattern Between Inner and Outer Distinct Chemical Pattern Between Inner and Outer Halo: Carbon-Enhanced Metal-Poor StarsHalo: Carbon-Enhanced Metal-Poor Stars

Sources of CarbonSources of Carbon

• Intrinsic internal production by low mass stars of extremely low [Fe/H] (Fujimoto et al 2000)

• Extrinsic production of carbon by stars of intermediate mass (2 < M < 8) during their AGB phase in a binary system (Suda et al. 2004)

• Faint Supernova Models: extensive mixing and fallback during explosions (Umeda & Nomoto, 2003; Iwamoto et al. 2005)

• Nucleosynthesis in Rotating Massive Stars (Meynet 2006, and references therein)

CEMP Stars in the Inner and Outer HaloCEMP Stars in the Inner and Outer HaloCarollo D. et al. 2012 ApJ, 744, 195Carollo D. et al. 2012 ApJ, 744, 195

Black Dots: Global trend of CEMPstellar fraction vs [Fe/H].

Blue dot-dashed curve: Second orderpolynomial fit.

Blue Filled Circles: Expected valuesof CEMP fraction in each bin of [Fe/H] (0.5 dex).

Green Filled Circle: Observed CEMPfraction for the inner halo.

Red Filled Circle: Observed CEMPfraction for the outer halo.

CEMP Stars: Summary

Observed increases of CEMP stars as [Fe/H] decreases

At -2.5 < [Fe/H] < -2.0 :

This trend could be confirmed at [Fe/H] < -2.5

HaloInnerHaloOuter CEMP2 CEMP

CEMP Stars: Implication for Galaxy Formation

One of the key element: Barium (Ba)

The majority of (~ 80%) CEMP stars exhibit over-abundances of Barium (CEMP-s stars: Beers & Christlieb 2005)

Ba is produced by s-process (n-capture) in AGB stars

A small fraction of CEMP stars have normal or low Ba abundances (CEMP-no)

CEMP-no: carbon excess perhaps has a different origin

CEMP-s stars are seen at [Fe/H] > -3.0 CEMP-no stars appear in the lowest metallicity range [Fe/H] < -2.5

CEMP Stars: Implication for Galaxy Formation

At -2.5 < [Fe/H] < -2.0 HaloInnerHaloOuter CEMP2 CEMP

Suggests that CEMP stars in the Outer Halo formed through different sources. Progenitors were predominantly massive fast rotating stars? Faint supernova?

CEMP stars in the inner halo formed predominantly in abinary system.

Is the CEMP-no fraction in the outer halo higher than in the inner halo?

Work still in progress (Carollo et al. in preparation)

CEMP Stars: connection with Ultra Faint dSphs

CEMP in the SEGUE 1 System(Norris, Gilmore, Wyse et al., 2010)

A

Nor

mal

ized

Flu

x

CEMP in the Halo

[Fe/H] = -3.52, [C/Fe] = +2.3; [Ba/Fe] < -1.0CEMP-no

Segue 1 and Bootes I:

Large Spread in Carbon

Abundance

CEMP Stars: connection with Ultra Faint dSphs

Bootes I observed with LRIS on Keck(Lai et al., 2011, Apj, 738, 51)

R = 2000, analyzed with the SSPP

[C/Fe] also estimated

Milky Way CDF at high |Z|

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Connection with galaxies at high redshift: Damped Ly Absorption Systems (DLAs)

• Recently a DLAs with [Fe/H] ~ -3 observed at z ~ 2.3 exhibit strong carbon enhancement (Cooke et al. 2011b).

• All others DLAs at [Fe/H] < -2 show chemical abundance ratios consistent with very metal poor Galactic halo stars (Cooke et al. 2011a).

Suggests possible connections between these high redshift galaxies and the early building blocks of the Milky Way –like galaxies.

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Summary

• The halo system of the Milky Way is extremely complex: a more clear picture is emerging in both observational and theoretical works.

• The smooth halo has at least two components which exhibit different kinematics, orbital parameters, spatial distribution and chemical composition.

• The duality of the Galactic halo is emerging in several observational results in which different sample of stars and techniques are used.

• The duality is dramatically emerging in high resolution simulation of Milky Way galaxies and it is a general property of such simulations.

• First clear evidence of different chemical pattern in the two components: the CEMP stars fraction

• DLAs : similar chemical patterns of the Milky Way’s stellar halos

HERMES is a new high-resolution fiber-fed multi-object spectrometer

on the AAT

Main driver: the GALAH survey (Galactic Archaeology with HERMES)

Team of about 40, mostly from Australian institutions

spectral resolution 28,000(also R = 50,000 mode)400 fibres over square degrees4 bands (BGRI) ~ 1000 ÅFirst light 2013

Use the detailed chemical abundances of stars to tag or associate them to common ancient star-forming aggregates with similar abundance patterns (eg Freeman & Bland-Hawthorn ARAA 2002)

The detailed abundance pattern reflects the chemical evolutionof the gas from which the aggregate formed.

Chemical Tagging

HERMES

The Galactic Halo(s) with HERMES

With HERMES ~ 50000 stars in the Halo

All these stars will be observed at a resolution of 28000 or more

Detailed abundance analysis for:

• Smooth Halos

• Substructures

My main scientific interests in the Galactic Archaeology Survey with HERMES

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While I’m waiting for the GALAH survey..

Test and validate:

• Data Reduction Pipeline

•Abundance Analysis Pipeline

The End

The Milky Way from Death Valley