Download Modelling the Stellar Populations of The Milky Way and Andromeda

Modelling the Stellar Populations of
The Milky Way and Andromeda
Collaborators:
Theory:
Kathryn Johnston (Columbia)
Brant Robertson (Chicago)
James Bullock (Irvine)
Observations:
Annette Ferguson (Edinburgh)
Puragra Guhathakurta (Santa Cruz)
Karrie Gilbert (Santa Cruz)
LCDM and stellar halos
Hierarchical structure formation leads to idea that stellar
halos formed from accreted, disrupted galaxies (~Searle &
Zinn). Stellar halo studies provide means to:
Measure accretion rates: Stellar streams around galaxies
Probe early star formation: Chemical abundance patterns.
Test CDM on small scales: Substructure counts
How does the Milky Way fit in the hierarchical picture?
Chemical Abundance Patterns
How do we make
this...
stellar halo
[α/Fe]
Venn et a. (2004)
... from
this?
dSph dIrr
satellite
data from Venn 04
stars
[Fe/H]
(Bullock & Johnston05)
A Hybrid Model:
1. Construct accretion histories for
Milky-Way type halos using semianalytic “merger tree”.
(Bullock & Johnston05)
A Hybrid Model:
1. Construct accretion histories for
Milky-Way type halos using semianalytic “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 semianalytic “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 semianalytic “merger tree”.
2. For each accreted system, model its
previous star formation history based on
expected mass growth history:
3. Initialize simulations, embed
stellar content into the center of
accreted dark matter halo to
match a realistic galaxy light
profile.
(Bullock & Johnston05)
A Hybrid Model:
1. Construct accretion histories for
Milky-Way type halos using semianalytic “merger tree”.
2. For each accreted system, model its
previous star formation history based on
expected mass growth history:
3. Embed stellars in the center
of accreted dark matter halo.
4. Follow evolution within the (growing)
host halo using basis function expansion
code. 100,000 particles per event.
Star formation, feedback and chemical model for
infalling satellites:
Gas / Dark Matter mass accretion
history:
Star formation law:
Only dwarf galaxies that formed
before reionization retain gas.
Blow-out Feedback Law:
Chemical Evolution Code:
N-body
simulations(Wechsler
et. al. 2002)
t* = 8Gyr:
Set to match
velocity-luminosity
relationship for
surviving satellites
Set to match
metallicity vs.
luminosity relation
for local group
dwarfs
Tracks Type Ia, Type II Supernovae (Robertson et al, 2005)
Font et al. 06a
Chemical evolution
models + N-body
simulations
Halo is alphaenhanced
because it is
formed from
earlier accretion
events.
Surviving
satellites were
accreted later.
halo
satellites
[Alpha/Fe] abundances
Satellite
galaxies:
intermediate
alpha
abundances
Venn et a. (2004)
[α/Fe]
Halo:
Enhanced alpha
abundances
(rapid star
formation)
data from Venn 04
[Fe/H]
Observations:
[Fe/H] gradients
Tanaka & Chiba
Predictions for
[Fe/H]
gradients
Font et al 2006b
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).