Modeling Star Formation and Chemical Evolution in the Local Group dwarfs Oleg Gnedin University of Michigan

Dolphin et al. 2005 and Holtzman et al. 2006 New detailed star formation histories of all classic Local Group dwarfs from archival HST CMDs: Dolphin et al. 2005 and Holtzman et al. 2006

Star formation histories of dSph and dIrr galaxies are similar dSph + dE dIrr + Tr

Star formation histories of dSph and dIrr galaxies are indistinguishable without the most recent 2 Gyr

Orban, OG, Weisz, Skillman, Dolphin & Holtzman ApJ, 686, 1030 (2008) Chris Orban (Ohio State) Dan Weisz and Evan Skillman (Minnesota)

(figure from Barkana & Loeb 2001) Primordial gas does not cool efficiently in halos with T < 104 K but we may also need to suppress star formation in larger halos Vc=10 km/s 100 km/s HI HeII H2 (figure from Barkana & Loeb 2001)

Different M/L relations have been proposed for selected halos to fit the luminosity function. We need complementary data to test the models: star formation histories and metallicities. SDSS number of dwarfs vs. velocity (proxy for mass) within 200 h-1 kpc of each large halo (MW and M31) Kravtsov, OG, Klypin (2004)

1 Gyr 2 Gyr 5 Gyr 10 Gyr stellar mass function mean age

Stochastic star formation is required to explain a small subset of relatively young stars (1-2 Gyr)

Stochastic star formation threshold allows recent SF Can determine the amount of variation

Example of stochastic star formation: isolated HII regions on the outskirts of spiral galaxies M83 with GALEX

Dynamical Mass within r = 0.3 kpc

Photometrically-derived abundances of heavy elements generally agree with those derived spectroscopically Mateo (2008)

Build-up of metallicity is tightly linked to growth of stellar mass  consistent with closed-box chemical evolution