The Local Group in Cosmological Context

1. The Local Group in Cosmological Context Rosemary Wyse Johns Hopkins University Subaru/NOAJ Symposium, Nov 2011

2. Exciting times to study local galaxies • Large observational surveys of stars in Local Group galaxies are feasible using wide-field imagers and multi-object spectroscopy, complemented by space-based imaging and spectroscopy, then Gaia and full phase space information • Important role for Subaru • There are copious numbers of stars nearby that have ages > 10 Gyr : formed at redshifts > 2 ~

3. Exciting times to study local galaxies • High-redshift surveys are now quantifying the stellar populations and morphologies of galaxies at high look-back times • Large, high-resolution simulations of structure formation are allowing predictions of Galaxy formation in a cosmological context, ΛCDM now, Warm Dark Matter soon

4. The Fossil Record: Galactic Archaeology • Complementary approach to direct study of galaxies at high redshift • Snapshots of different galaxies vs evolution of same galaxy • Derive metallicity and elemental abundance distributions, and age distributions….separately • break degeneracies of integrated light • Stellar IMF • Kinematics of stars to dynamics, dark matter

5. ΛCDM cosmology extremely successful on large scales. Galaxies are the scales on which one must see thenature of dark matter & astrophysics Ostriker & Steinhardt 03 Inner DM mass density depends on the type(s) of DM Galaxy mass function depends on DM type

6. ‘MW’ Dark Halo in ΛCDM:(Far Too) Much Substructure GHALO (Stadel et al 09)

7. ΛCDM: Hierarchical clustering • Merging very important in evolution of Milky Way mass systems, to late times • Merging builds up bulges (both stars and gas) and heats thin stellar disks, can destroy (re-formed later), add gas and stars (all ages) • Angular momentum transport causes compact disks • Low-density/low mass systems disrupted early can form stellar halo

8. Thick stellar disks and massive bulges: Edge-on projected present-day stellar luminosity distributions from a suite of SPH simulations of Milky Way-mass galaxies in ΛCDM (Scannipieco et al. 2011; see also House et al 2011) • Mergers also re-arrange thin disk radially • Migration, maintaining circular orbits, plus heating

9. Radial Migration • Transient gravitational perturbations can cause stars and gas in circular orbits at corotation to migrate radially in disk (Sellwood & Binney 2002) • Additional effects from bar/spiral interaction (Minchev et al. 2011) • Proposed as mechanism to form thick disk without any mergers (e.g. Schönrich & Binney 2009) • Efficiency for stars on non-circular orbits? • How to produce/maintain very narrow iron abundance distribution at given radius?

10. How to form a disk galaxy like the MW? • Generic disk galaxy in ΛCDM has large bulge-disk ratio and active merging history • Select atypical Galaxy-mass halo with no significant (1:10) merging since redshift of three, re-simulate at high-resolution using SPH (Guedes et al 2011; Eris) • But late-type (Sb/Sbc) disk galaxies are not rare! • Suppress early star formation through high gas density threshold • Few old stars in disk at two scale-lengths now • Strong feedback in central regions to remove low angular momentum gas • Potential well assembled (no merging) so needs to be very strong

11. Stellar Halo • In ΛCDM, stellar halo forms from stars of disrupted subhaloes • Satellite galaxies from surviving subhaloes Johnston et al 08

12. Dark-matter halos in ΛCDM have ‘cusped’ density profiles Diemand et al 08 Continually varying power-law (Einasto profile) ραr -1 in inner regions Test best in systems with least contribution to mass from baryons : dwarf spheroidal galaxies Main halo Sub-halos Lower limits here

13. The Milky Way and M31 as templates • Stellar halo, bulge, thick disk and even some part of (old?) thin disk predicted to be created through mergers • Should see signatures in stellar populations • Stars retain memory of conditions when formed • Coordinate space structure • Kinematic (sub)structure • Chemical signatures: self-enrichment and massive-star IMF • Age distributions date merger • Satellite galaxies and streams from deep, uniform imaging, followed by spectroscopy

14. Satellite stellar content: • Orders of magnitude discrepancy in number compared to dark subhaloes in ΛCDM • Suppress star formation • Cannot simultaneously fit LMC and fainter • Stellar populations, spatial distribution also remain problems

15. Semi-analytic models (left, Koposov et al 2008; right, Rashkov et al 2011) cannot fit luminosity function for all luminosities – problem with LMC • Cannot appeal to variable stellar IMF • LMC also very blue—on first orbit? • Why stream so long? (Nidever et al 2010)

16. Main sequence luminosity functions of UMi dSph and of globular clusters are indistinguishable.  normal low-mass IMF at 10-12Gyr lookback HST star counts Wyse et al 2002 Massive-star IMF constrained by elemental abundances – also normal M92  M15  0.3M

17. Gilmore et al; Norris et al 10 BooI Boo I  Simon et al 10 Leo IV dSphs vs. MWG abundances(from A. Koch, 2009 + updates) Frebel et al 10 Scl Leo IV ◊ Scl        Shetrone et al. (2001, 2003): 5 dSphs Koch et al. (2008): Hercules Sadakane et al. (2004): Ursa Minor Shetrone et al. (2008): Leo II Monaco et al. (2005): Sagittarius Frebel et al. (2009): Coma Ber, Ursa Major Koch et al. (2006, 2007): Carina Aoki et al. (2009): Sextans Letarte (2006): Fornax Hill et al. (in prep): Sculptor

18. Same ‘plateau’ in [α/Fe] in all systems at lowest metallicities • Type II enrichment only: massive-star IMF invariant, and well-sampled – good mixing required • Stellar halo could form from any system(s) in which star-formation is short-lived, and inefficient so that mean metallicity kept low, ISM well-mixed • Star clusters, galaxies, transient structures… • Complementary, independent age information that bulk of halo stars are OLD further constrains progenitors (e.g. Unavane, Wyse & Gilmore 1996)

19. Spectroscopy of luminous dSph  kinematics  Velocity dispersion profile  mass Gilmore et al 07; see also Walker et al 07; Walker et al 09; 11 1 Isotropic Jeans analysis: Very dark-matter dominated; all dSph similar, favour cores, not CDM cusp. Range of (low) star-formation histories, hard to re-arrange all by feedback. WDM instead? Same from gas-rich dwarfs) 0.1 ρ (M/pc3) Full DF modelling underway larger stellar samples 0.1 R (kpc) 1

20. Thick Disk • Clear identification by vertical star-counts – two exponentials fit and one does not • Stars predominantly old, 10-12Gyr • Thick disk globular clusters are as old as the stellar halo globulars • Turn-off age also same as stellar halo • Old age of thick disk unusual in CDM; requires only ancient mergers to heat thin stellar disk • 95% of 1012M have merger with 5x1010M (=Mdisk ) in last 10Gyr (Stewart et al 08)

21. If early (minor) merger heated a pre-existing thin disk to create thick disk, and that thin disk had (expected) lower star formation rates in outer parts, thick disk in outer parts could well have lower [α/Fe] than in inner parts/solar neighborhood – shortest delay time for Type Ia less than 1Gyr, perhaps 100Myr  could be dangerous to identify`thick disk’ by invariant elemental abundances, far from Sun in Galactocentric radius How best to define?

22. Early thick disks will be compressed and heated by accretion/re-formation of thin disk (Ostriker 1990; Elmegreen & Elmegreen 2006) • Adiabatic growth would lead to ΔH/H ~ - ΔMgas/Mdisk Δσ2/σ2 ~ -2 ΔH/H • Clumpy turbulent disks at redshift ~ 2 may form bulges

23. Old stars everywhere…ages 10-12Gyr • Bulge beyond 300pc is old, narrow age range • Thick disk is old (within several kpc of sun), narrow age range • Local thin disk contains old stars • See well-formed disks at redshift 2, look-back 10Gyr • Formed in situ? • Satellite galaxies all contain old stars, as old as could be detected  not natural in ΛCDM  WDM?

24. Concluding remarks • Baryon physics critical to understanding dark matter, particularly on galaxy scales • Resolved stellar populations unique role • Many aspects of Local Group galaxies pose challenges for current paradigm • `More high quality data for carefully selected samples are needed, plus well-motivated robust models’