1. Modeling Star Formation Histories of Local Group Dwarfs
Chris Orban
Center for Cosmology and
Astro-Particle Physics
The Ohio State University
with
Oleg Gnedin (UMich)
Globular Clusters – Dwarf Galaxies Connection Workshop

2. Missing Satellites Problem
Kravtsov, Gnedin & Klypin 2004
Mass 
Vmax = (GM/R)1/2
About 10% of the small DM halos at
z = 0 had LMC/SMC-sized halos in the past
Strong tidal interactions cause dramatic mass loss
Globular Clusters – Dwarf Galaxies Connection Workshop

3. Radial Distributions
All Dwarf Galaxies
dSph and dE Type Galaxies
Globular Clusters – Dwarf Galaxies Connection Workshop

4. New Data! Color-Magnitude Diagrams Star Formation Histories
SFR (Msun/yr)
Time 
Dolphin, Weisz, Skillman & Holtzman 2005
Holtzman, Afonso, & Dolphin, 2006
Globular Clusters – Dwarf Galaxies Connection Workshop

5. An Example: New Data for Leo I Dwarf
Star Formation Rate
Cumulative Star Formation
fxG= fraction of all stars formed in the last xGyr
Globular Clusters – Dwarf Galaxies Connection Workshop

6. Star Formation Model exp(-r/rd), =
from Kravtsov, Gnedin & Klypin 2004
Each halo is assumed to have a gas disk, =
exp(-r/rd),
Star Formation Rate (Msun/yr)
for > = 5 Msun/pc2, (Kennicutt 1998)
Also include starbursts and UV background ionization (Gnedin 2000)
Globular Clusters – Dwarf Galaxies Connection Workshop

7. A New Addition! A stochastic star formation threshold
Allows the density threshold for the Schmidt law to vary
In the models shown here, e* = 0.1
Globular Clusters – Dwarf Galaxies Connection Workshop

9. Globular Clusters – Dwarf Galaxies Connection Workshop

10. Reionization effects in f10G?
General picture is that dwarfs we see today were in larger halos which held more gas and formed more stars early on.
Can improve f10G agreement if the dwarfs without early star formation (<1Gyr) are rejected.
Implies gas accretion is (more) suppressed in low mass halos at early times
Globular Clusters – Dwarf Galaxies Connection Workshop

11. f10G

12. Globular Clusters – Dwarf Galaxies Connection Workshop

13. Conclusions
Kennicutt-Schmidt law still consistent with dwarf galaxy star formation histories if one includes a stochastic density threshold.
The f10G fit is improved if non-early star formers are rejected (possible effect of reionization).
Not (quite) enough late star formation.
Globular Clusters – Dwarf Galaxies Connection Workshop