IAU General Assembly 2009 Symposium Daniela Carollo Macquarie University Research Centre in Astronomy, Astrophysics & Astrophotonics Department of Physics & Astronomy, Macquarie University Aboriginal Art Milky Way Dreaming G. Possum The Current View of the Halo System of the Milky Way

The Galactic Stellar HaloLocus of Ancient Stars The Galactic Stellar Halo: Locus of Ancient Stars Their motion has an imprints of the dynamical history of the Galaxy chemical composition Their chemical 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

3 The halo system Smooth components: Inner and Outer Halo Substructures Overdensities Globular Clusters

 Inner Halo:  Dominant at R < kpc  Highly eccentric orbits  Slightly Prograde  Metallicity peak at [Fe/H] = -1.6  Outer Halo:  Dominant at R > kpc  More uniform distribution of eccentricity  Highly retrograde orbits  Metallicity peak around [Fe/H] = -2.2 Smooth Halo Components D.Carollo et al., Nature 2007, Vol. 450, D. Carollo et al., Nature 2007, Vol. 450, Halo  Halos

Stellar Fractions, F IN and F OUT, as a function of Z max The value Z max ~ kpc is the inversion point! Full [Fe/H] Range Extracting the Fractions of Each Galactic Component Carollo et al. 2010, ApJ, 712, 692

Inner and Outer Halo Velocity Ellipsoids Inner Halo : Ellipsoid dominated by the radial motion Outer Halo: Ellipsoid less dominated by the radial motion Outer Halo: Ellipsoid less dominated by the radial motion Carollo et al. 2010, ApJ, 712, 692

Power-Law Density Profiles for Inner/Outer Halo 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? It is not just that it is double… Members of the two components exhibit: Different Spatial Distributions Different Kinematics Different Chemical Composition (MDFs) Distinct Astrophysical Origin Crucial to Understand the Formation and Subsequent Evolution of the Milky Way Inner Halo Inner Halo : Dissipative radial merger of few massive sub-Galactic fragments formed at early stage, mainly in situ stars Outer Halo Outer 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 82 (An et al., submitted) Observed photometric MDF is inconsistent with a single [Fe/H] population.

Distinct Halo Populations in the Solar Neighborhood Nissen & Schuster, 2010 Nissen & Schuster, 2010 High [  /Fe]Prograde Orbits High [  /Fe] Mainly Prograde Orbits Low [  /Fe]Retrograde Orbits Low [  /Fe] Mainly Retrograde 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) 11 Metallicity shift as a function of the Galactocentric distance: [Fe/H] ~ -1.6 at R kpc Mean stellar mass density exhibit two halo populations with inversion points in the predominance at kpc

12 Disk/Halos Structure from SEGUE Stellar Photometry (De Jong et al. 2010) (De Jong et al. 2010) Left Panel Black dots: total stellar mass density Red line: density for the thick-disk- like population Blue line: density for the inner- halo- like population Green line: density for the outer- halo-like population Right Panel Fractional contribution of the individual template populations Inner-outer halo inversion point between kpc. In agreement with Carollo et al. ( ).

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

14 The Dual Halo in the High Resolution Simulations Font et al 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

15 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] 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] ~ CEMP stars contain useful information on the nature of nucleosynthesis in the early Galaxy. Where does this carbon come from? Distinct Chemical Pattern Between Inner and Outer Halo: Carbon-Enhanced Metal-Poor Stars

Sources 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 Halo Carollo D. et al ApJ, 744, 195 Black Dots: Global trend of CEMP stellar fraction vs [Fe/H]. Blue dot-dashed curve: Second order polynomial fit. Blue Filled Circles: Expected values of CEMP fraction in each bin of [Fe/H] (0.5 dex). Green Filled Circle: Observed CEMP fraction for the inner halo. Red Filled Circle: Observed CEMP fraction 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

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 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 a binary 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 Normalized Flux CEMP in the Halo [Fe/H] = -3.52, [C/Fe] = +2.3; [Ba/Fe] < -1.0 CEMP-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|

24 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.

25 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 degrees 4 bands (BGRI) ~ 1000 Å First 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 evolution of the gas from which the aggregate formed. Chemical Tagging HERMES

The Galactic Halo(s) with HERMES With HERMES ~ stars in the Halo All these stars will be observed at a resolution of or more Detailed abundance analysis for: Smooth Halos Substructures My main scientific interests in the Galactic Archaeology Survey with HERMES

29 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