Modelling the Stellar Populations of The Milky Way and 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/Fe] [Fe/H] How does the Milky Way fit in the hierarchical picture? How do we make this from this? Chemical Abundance Patterns stellar halo dIrr dSph dIrr satellite 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 on expected 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(Wechsler 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 formed before 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/Fe]

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.

Conclusions 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).