Exploring the origin of the stellar halo of the Milky Way

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Exploring the origin of the stellar halo of the
Milky Way

• Eric Bell
• Ann Arbor 29 July 2009

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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
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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

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Sloan Digital Sky Survey

• ugriz imaging 10000 sq. degrees r23
• Spectroscopy for 106 gals/stars r17.7

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Study of halos

• Color-magnitude diagrams
• Selection of standard candle stars

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Thick disk turn-off stars
Foreground Disk low-mass stars
Halo turn-off stars
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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
? r-2.5 - r-3
log ?
? r-3 - r-3.5
? r-3
r 20kpc
c/a 0.6 i.e., stellar halo is oblate
log r
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Results

• best halo fit is r-3, between 5ltrgc/kpclt40
  perhaps shallower inside 20kpc and steeper
  outside
• Oblate 0.5 lt c/a lt 0.8
• Is smooth model a good fit?
• RMS of data around model take off Poisson in
  quadrature from RMS

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Results

• best halo fit is r-3, between 5ltrgc/kpclt40
  perhaps shallower inside 20kpc and steeper
  outside
• Oblate 0.5 lt c/a lt 0.8
• BUT, smooth model is a poor fit RMS/total gt
  0.4
• Indications of more structure
  at larger radii

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Comparison with simulations I

• Results so far
• r-3 halo at lt40kpc
• 0.5 lt c/a lt 0.8
• M 3.5/-1.5 x 108 Msun
• Smooth halo poor fit
• Are these properties consistent with being built
  up through accretion alone?

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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

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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
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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
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BHB
MSTO
BHB/MSTO

• BHBs convolved with MSTO distance uncertainties
• BHB distribution richly-structured
• Different halo structures have distinctive
  populations

Bell, Xue, Ruhland, Rix, Hogg 2009
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• 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
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NGC 5907 Martinez-Delgado et al. 2008 NGC 4013
M31 stellar halo Very richly structured Ferguson
et al. 2003 Ibata et al. 2002..07 Zucker et al.
2004
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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)