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

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

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

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

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

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

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

Globular Clusters – Dwarf Galaxies Connection Workshop

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

f10G

Globular Clusters – Dwarf Galaxies Connection Workshop

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