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.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

29 July 2009 UM Ann ArborBell et al.

Stellar halo : fossil record of assembly?

Dwarf galaxies are disrupting and contributing to the stellar halo 1% of stellar mass

QuickTime™ and aCinepak decompressor

are needed to see this picture.

Bullock & Johnston 2005Bullock & Johnston 2005See also Ibata et al. 1994, 1995See also Ibata et al. 1994, 1995

Majewski et al. 2003, Majewski et al. 2003, Martinez-Delgado et al. 2004Martinez-Delgado et al. 2004

Belokurov et al. 2006Belokurov et al. 2006

29 July 2009 UM Ann ArborBell 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

29 July 2009 UM Ann ArborBell et al.

Sloan Digital Sky Survey

ugriz imaging ~10000 sq. degrees r~23

Spectroscopy for ~106 gals/stars r~17.7

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

29 July 2009 UM Ann ArborBell et al.

Study of halos

Color-magnitude diagrams Selection of ‘standard candle’ stars

29 July 2009 UM Ann ArborBell et al.

ForegroundForegroundDisk low-mass starsDisk low-mass stars

Thick disk turn-off starsThick disk turn-off stars

Halo turn-off Halo turn-off starsstars

29 July 2009 UM Ann ArborBell et al.

29 July 2009 UM Ann ArborBell et al.

log log ρρ

log rlog r

r ~ 20kpcr ~ 20kpc

A smooth halo model

Previous resultsPrevious results

Morrison et al. 2000Morrison et al. 2000Chiba & Beers 2000Chiba & Beers 2000

Ivesic et al. 2000Ivesic et al. 2000Lemon et al. 2004Lemon et al. 2004

Xu et al. 2006Xu et al. 2006ρρ ~ r ~ r-3-3 - r - r-3.5 -3.5

ρρ ~ r ~ r-2.5-2.5 - r - r-3 -3

ρρ ~ r ~ r-3 -3

c/a ~ 0.6c/a ~ 0.6i.e., stellar halo is oblatei.e., stellar halo is oblate

29 July 2009 UM Ann ArborBell et al.

Results

‘best’ halo fit is ~r-3, between 5<rgc/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

29 July 2009 UM Ann ArborBell et al.

Results

‘best’ halo fit is ~r-3, between 5<rgc/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

29 July 2009 UM Ann ArborBell et al.

29 July 2009 UM Ann ArborBell 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 108

Msun

Smooth halo poor fit

Are these properties consistent with being built up through accretion alone?

29 July 2009 UM Ann ArborBell 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

29 July 2009 UM Ann ArborBell 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. 2008Bell et al. 2008

29 July 2009 UM Ann ArborBell et al.

Population variationsin the stellar halo

Probe for stellar pops.differences using BHB stars

Use ugr colors to select high probability BHB candidates

Bell, Xue, Ruhland, Bell, Xue, Ruhland, Rix, Hogg 2009Rix, Hogg 2009

29 July 2009 UM Ann ArborBell et al.

BHBs convolved with MSTO distance uncertainties

BHB distribution richly-structured

Different halo structures have distinctive populations

BHBBHB MSTOMSTO BHB/MSTOBHB/MSTO

Bell, Xue, Ruhland, Bell, Xue, Ruhland, Rix, Hogg 2009Rix, Hogg 2009

29 July 2009 UM Ann ArborBell 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, Bell, Xue, Ruhland, Rix, Hogg 2009Rix, Hogg 2009

29 July 2009 UM Ann ArborBell et al.

M31 stellar haloM31 stellar halo

Very richly structuredVery richly structured

Ferguson et al. 2003Ferguson et al. 2003Ibata et al. 2002..07Ibata et al. 2002..07Zucker et al. 2004Zucker et al. 2004

NGC 5907NGC 5907

Martinez-Delgado et al. 2008Martinez-Delgado et al. 2008

NGC 4013NGC 4013

29 July 2009 UM Ann ArborBell 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)