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

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

7. ΛCDM: Hierarchical clustering ΛCDM: Hierarchical clustering Merging very important in evolution of Milky Way mass systems, to late times 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) 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 Angular momentum transport causes compact disks Low-density/low mass systems disrupted early can form stellar halo 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  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) 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) 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) Proposed as mechanism to form thick disk without any mergers (e.g. Schönrich & Binney 2009) Efficiency for stars on non-circular orbits? Efficiency for stars on non-circular orbits? How to produce/maintain very narrow iron abundance distribution at given radius? 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 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) 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! But late-type (Sb/Sbc) disk galaxies are not rare! Suppress early star formation through high gas density threshold Suppress early star formation through high gas density threshold Few old stars in disk at two scale-lengths now Few old stars in disk at two scale-lengths now Strong feedback in central regions to remove low angular momentum gas Strong feedback in central regions to remove low angular momentum gas Potential well assembled (no merging) so needs to be very strong Potential well assembled (no merging) so needs to be very strong

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

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

13.  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 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 The Milky Way and M31 as templates

14. Satellite stellar content: Orders of magnitude discrepancy in number compared to dark subhaloes in ΛCDM 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 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 Cannot appeal to variable stellar IMF LMC also very blue—on first orbit? LMC also very blue—on first orbit? Why stream so long? (Nidever et al 2010) Why stream so long? (Nidever et al 2010)

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

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

18. Same ‘plateau’ in [ α /Fe] in all systems at lowest metallicities 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 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 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… 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) 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 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) Full DF modelling underway larger stellar samples Gilmore et al 07; see also Walker et al 07; Walker et al 09; 11 0.1 R (kpc) 1 ρ (M  /pc 3) 1 0.1

20. Thick Disk Clear identification by vertical star-counts – two exponentials fit and one does not Clear identification by vertical star-counts – two exponentials fit and one does not Stars predominantly old, 10-12Gyr Stars predominantly old, 10-12Gyr Thick disk globular clusters are as old as the stellar halo globulars Thick disk globular clusters are as old as the stellar halo globulars Turn-off age also same as stellar halo 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 10 12 M  have merger with 5x10 10 M  (=M disk ) in last 10Gyr (Stewart et al 08)

21. How best to define? 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 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  could be dangerous to identify`thick disk’ by invariant elemental abundances, far from Sun in Galactocentric radius

22. Early thick disks will be compressed and heated by accretion/re-formation of thin disk (Ostriker 1990; Elmegreen & Elmegreen 2006) 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 Adiabatic growth would lead to ΔH/H ~ - ΔM gas /M disk ΔH/H ~ - ΔM gas /M disk Δσ 2 /σ 2 ~ -2 ΔH/H Δσ 2 /σ 2 ~ -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 Bulge beyond 300pc is old, narrow age range Thick disk is old (within several kpc of sun), narrow age range Thick disk is old (within several kpc of sun), narrow age range Local thin disk contains old stars Local thin disk contains old stars See well-formed disks at redshift 2, look-back 10Gyr See well-formed disks at redshift 2, look-back 10Gyr Formed in situ? Formed in situ? Satellite galaxies all contain old stars, as old as could be detected Satellite galaxies all contain old stars, as old as could be detected  not natural in ΛCDM  WDM?  not natural in ΛCDM  WDM?

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