1. 1 Exploring the origin of the stellar halo of the Milky Way Eric Bell Ann Arbor 29 July 2009 Eric Bell Ann Arbor 29 July 2009

2. 29 July 2009 UM Ann Arbor Bell et al. Stellar halo : fossil record of assembly?  Dwarf galaxies are disrupting and contributing to the stellar halo  1% of stellar mass Bullock & Johnston 2005 See also Ibata et al. 1994, 1995 Majewski et al. 2003, Martinez-Delgado et al. 2004 Belokurov et al. 2006

3. 29 July 2009 UM Ann Arbor Bell et al. The question  LCDM predicts that stellar halo primarily accreted  Hierarchical build-up of stellar halos clearly happens, but  Is it icing on the cake?  Minor addition to halo mostly built up before  Or is it the cake?  Major addition to halo

4. 29 July 2009 UM Ann Arbor Bell et al. Sloan Digital Sky Survey  ugriz imaging ~10000 sq. degrees r~23  Spectroscopy for ~10 6 gals/stars r~17.7

5. 29 July 2009 UM Ann Arbor Bell et al. Study of halos  Color-magnitude diagrams  Selection of ‘standard candle’ stars

6. 29 July 2009 UM Ann Arbor Bell et al. Foreground Disk low-mass stars Thick disk turn-off stars Halo turn-off stars

7. 29 July 2009 UM Ann Arbor Bell et al.

8. 29 July 2009 UM Ann Arbor Bell et al. log ρ log r r ~ 20kpc A smooth halo model Previous results Morrison et al. 2000 Chiba & Beers 2000 Ivesic et al. 2000 Lemon et al. 2004 Xu et al. 2006 ρ ~ r -3 - r -3.5 ρ ~ r -2.5 - r -3 ρ ~ r -3 c/a ~ 0.6 i.e., stellar halo is oblate

9. 29 July 2009 UM Ann Arbor Bell et al. Results  ‘best’ halo fit is ~r -3, between 5<r gc /kpc<40; perhaps shallower inside 20kpc and steeper outside  Oblate 0.5 < c/a < 0.8  Is smooth model a good fit?  RMS of data around model; take off Poisson in quadrature from RMS

10. 29 July 2009 UM Ann Arbor Bell et al. Results  ‘best’ halo fit is ~r -3, between 5<r gc /kpc<40; perhaps shallower inside 20kpc and steeper outside  Oblate 0.5 < c/a < 0.8  BUT, smooth model is a poor fit RMS/total > 0.4  Indications of more structure at larger radii

11. 29 July 2009 UM Ann Arbor Bell et al.

12. 29 July 2009 UM Ann Arbor Bell et al. Comparison with simulations I  Results so far:  r -3 halo at <~40kpc  0.5 < c/a < 0.8  M ~ 3.5+/-1.5 x 10 8 M sun  Smooth halo poor fit  Are these properties consistent with being built up through accretion alone?

13. 29 July 2009 UM Ann Arbor Bell et al. Comparison with simulations II  Models build up stellar halo only through accretion  Satellite population realistic (assumed solution to sub- structure problem)  Bullock & Johnston 2005  Signficant model-to- model scatter  Consistent with observations

14. 29 July 2009 UM Ann Arbor Bell et al. Comparison with simulations III  Structure of residuals similar to the data  Milky Way halo consistent with being built up through accretion alone Bell et al. 2008

15. 29 July 2009 UM Ann Arbor Bell et al. Population variations in the stellar halo Probe for stellar pops. differences using BHB stars Use ugr colors to select high probability BHB candidates Bell, Xue, Ruhland, Rix, Hogg 2009

16. 29 July 2009 UM Ann Arbor Bell et al.  BHBs convolved with MSTO distance uncertainties  BHB distribution richly-structured  Different halo structures have distinctive populations BHBMSTOBHB/MSTO Bell, Xue, Ruhland, Rix, Hogg 2009

17. 29 July 2009 UM Ann Arbor Bell et al. Comparison with 3 simulated halos (Bullock & Johnston 2005)  Color-coded by metallicity  Coherent population variations  Data is in qualitative agreement with expectations from simulations Bell, Xue, Ruhland, Rix, Hogg 2009

18. 29 July 2009 UM Ann Arbor Bell et al. M31 stellar halo Very richly structured Ferguson et al. 2003 Ibata et al. 2002..07 Zucker et al. 2004 NGC 5907 Martinez-Delgado et al. 2008 NGC 4013

19. 29 July 2009 UM Ann Arbor Bell et al. Stellar halos  Results  Stellar halos richly structured  Coherent population variations  Quantitatively consistent with simulations in which stellar halos are formed through tidal disruption of galaxies  Features of this picture  In ~MW mass galaxies, bulk of stellar halo comes from largest progenitors  All galaxies have stellar halos; properties set by how dark halos are populated by stars (Purcell et al. 07)