Modelling the Stellar Populations of The Milky Way and Andromeda Collaborators: Theory:Observations:...
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Transcript of Modelling the Stellar Populations of The Milky Way and Andromeda Collaborators: Theory:Observations:...
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).