Modelling the Stellar Modelling the Stellar Populations of The Milky Way Populations of The Milky Way

and Andromedaand Andromeda

Collaborators:Theory: Observations:

Kathryn Johnston (Columbia) Annette Ferguson (Edinburgh)Brant Robertson (Chicago) Puragra Guhathakurta (Santa Cruz) James Bullock (Irvine) Karrie Gilbert (Santa Cruz)

LCDM and stellar halos

Measure accretion rates: Stellar streams around galaxies

Probe early star formation: Chemical abundance patterns.

Test CDM on small scales: Substructure counts

Hierarchical structure formation leads to idea that stellar halos formed from accreted, disrupted galaxies (~Searle & Zinn). Stellar halo studies provide means to:

Venn et a. (2004)

data from Venn 04

[α/F

e]

[Fe/H]

How does the Milky Way fit in the hierarchical picture?

How do we make this...

... from this?

Chemical Abundance Patterns

stellar halo

dSph dIrrdIrrsatellite

stars

A Hybrid Model:1. Construct accretion histories for Milky-Way type halos using semi-analytic “merger tree”.

(Bullock & Johnston05)

A Hybrid Model:1. Construct accretion histories for Milky-Way type halos using semi-analytic “merger tree”.

2. For each accreted system, model its previous star formation history based on expected mass growth history:

(Bullock & Johnston05)

A Hybrid Model:1. Construct accretion histories for Milky-Way type halos using semi-analytic “merger tree”.

2. For each accreted system, model its previous star formation history based on expected mass growth history:

(Bullock & Johnston05)

A Hybrid Model:

3. Initialize simulations, embed stellar content into the center of accreted dark matter halo to match a realistic galaxy light profile.

1. Construct accretion histories for Milky-Way type halos using semi-analytic “merger tree”.

2. For each accreted system, model its previous star formation history based on expected mass growth history:

(Bullock & Johnston05)

A Hybrid Model:

3. Embed stellars in the center of accreted dark matter halo.

1. Construct accretion histories for Milky-Way type halos using semi-analytic “merger tree”.

2. For each accreted system, model its previous star formation history based onexpected mass growth history:

4. Follow evolution within the (growing) host halo using basis function expansion code. 100,000 particles per event.

(Bullock & Johnston05)

N-body simulations(Wechsl

er et. al. 2002)

t* = 8Gyr:Set to match velocity-luminosity relationship for surviving satellites

Star formation, feedback and chemical model for

infalling satellites:Gas / Dark Matter mass accretion history:

Star formation law:

Blow-out Feedback Law:

Set to match metallicity vs. luminosity relation for local group dwarfs

Only dwarf galaxies that formedbefore reionization retain gas.

Chemical Evolution Code:Tracks Type Ia, Type II Supernovae (Robertson et al, 2005)

Font et al. 06a

Halo is alpha-enhanced because it is formed from earlier accretion events. Surviving satellites were accreted later.

Chemical evolution models + N-body

simulations halo

satellites

Venn et a. (2004)

data from Venn 04

[Fe/H]

[Alpha/Fe] abundances

Halo:Enhanced

alpha abundances (rapid star formation)

Satellite galaxies:

intermediate alpha

abundances

[α/F

e]

Tanaka & Chiba

Observations:

[Fe/H] gradients

Font et al 2006b

Predictions for [Fe/H] gradients

How does M31 fit in the hierarchical picture?

Font et al 2007

Halo Data(Brown et al 2007)

Halo formed btw 8-12 Gyr ago, some intermediate age populations added by a 1-2 massive satellites accreted recently (6-8 Gyr ago)

How does M31 fit in the hierarchical picture?

Font et al 2007

Stream Data(Brown et al 2007)

The progenitor of the Giant Stream likely to be a massive dwarf galaxy accreted 6-7 Gyr ago.

ConclusionsConclusions Models for the stellar halos based within the LCDM context can reproduce the gross characteristics of the MWy stellar halo and local group satellites. Chemical Abundance Patterns seem to arise naturally in this context.

Surveys are underway to test whether the stellar halos of the Milky Way and other nearby galaxies look like this... test whether structure formation is indeed hierarchical on small scales (eg. Bell et al 07, Ibata et al 07).

Surveys are underway to test whether the stellar halos of the Milky Way and other nearby galaxies look like this... test whether structure formation is indeed hierarchical on small scales (eg. Bell et al 07, Ibata et al 07).