Kathryn Johnston

Stellar
Halos and
Satellite
Systems in
a Lambda
CDM
Universe
Kathryn V Johnston (Wesleyan)
James S Bullock (Irvine)
Andreea Font (Wesleyan)
Brant Robertson (Harvard)
Bullock & Johnston, 2005
Robertson et al, 2005
Font et al, 2005

Given:
Motivation
– hierarchical structure formation;
– properties of Local Group dwarfs
What can we say about:
– stellar halos
– satellite systems
– substructure in stellar halos
(NGC5907, Shang et al 1999)
(inspired by Majewski, Spaghetti, SDSS, Ibata et al, Guhathakurta....)
Modeling (and observational) problem: low
luminosity and surface brightness of objects
and features we’re interested in.
(related studies: Brook et al; Knebe et al; Rende et al; Abadi, Navarro,
Steinmetz; Diemand, Madau, Moore....)

Dark matter
modeled via
merger tree.
100,000-particle
N-body model
run for each
luminous satellite
accreted.
Masses, orbits
accretion times
dictated by
cosmology
Methods - Our Approach
time
Light matter
painted on
subsequently
to match
properties of
Local Group
dwarfs today....

Methods - Sanity Checks
- 11 satellites of Milky Way (similar number for M31)
=> how many baryons to assign to each dark satellite (via
“squelching” of low end of luminosity function due to re-ionization -
see Bullock, Kravtsov & Weinberg, 2000)
- field dwarfs have significant gas content (e.g. Grebel et al , 2003)
=> how fast to turn baryons into stars (i.e. long timescale for star
formation, consistent with earlier talks...)
- field dwarfs follow well defined correlations in luminosity and core
radius (e.g. Mateo, 1998)
=> how to embed stars within halos
In addition, the model fits:
(i) stellar halo luminosity;
(ii) Hernquist profile;
(iii) distribution of satellite structural properties.

Model for Formation of a Halo
Color bar 34-24 mag/arcsec 2
Luminosity-weighted
dark matter
Embedded King models

Model for Formation of a Halo
Color bar 38-23 mag/arcsec^2, boxsize 300kpc
(cf: Pieter van Dokkum’s images)

Features
brighter than
30 mag/
arcsec^2
common

Brightest
features
generally
correspond
to rare,
latedisruption
events

Model for Formation of a Halo
- Halo built inside
out
- Majority of halo
from few most
massive events
- Surviving
satellites accreted
recently
- Mostly smaller
than those that
built the halo

Abundance Patterns in Satellites vs
Venn et al (2004)
Halo
The challenge for
Lambda CDM:
satellites look
chemically different
from the halo....
(e.g. Unnavane,
Wyse & Gilmore,
1996)

Abundance Patterns in Satellites vs
Halo
You cannot get ....
from....
...*!@!!!!

Abundance Patterns in Satellites vs
Created in SNII
< 100 Myr timescale
Created in SNII
and SNIa
> 1 Gyr timescale
Halo

Abundance Patterns in Satellites
vs Halo
Test our timing/mass arguments:
• Brant Robertson: leaky/accreting box chemical
evolution code (Robertson, Bullock, Font, Johnston &
Hernquist, 2005) requiring matches to
- mass-metallicity relation for dwarfs today
- low alpha-element abundances of dwarfs today
• Andreea Font: combine this with our results to create
phase-space plus abundance space models (Font,
Johnston, Bullock & Robertson, 2005).

Boxsize:
300kpc
38-23
mag/arcsec^2
-2.5 < [Fe/H]
< -0.5
-0.1 < [alpha/Fe]
< 0.2

Abundance Patterns in Satellites
vs Halo

Abundance
variations
within
stellar halos
e.g M31 -
Ferguson, Ibata,
Irwin, Chapman,
Lewis....
(see also Guhathakurta;
Brown et al; Mouchine et
al...)

... only stars
with [alpha/
Fe]

Summary
The BIG picture: (i) stellar halos are built inside
out from a few massive dwarfs; (ii) bulk of halo
accreted much earlier than surviving satellites.
Natural consequences of Lambda CDM:
=> features from recent events at ~30 mag/arcsec^2
common
=> these represent relatively rare, late-infall events
=> abundance patterns in late accretions and
surviving satellites different from rest of halo
=> cuts in alpha-abundance will be sensitive to
different accretion times