Modeling Star Formation Histories of Local Group Dwarfs

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 • • 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 Mass Vmax = (GM/R)1/2 Globular Clusters – Dwarf Galaxies Connection Workshop

Radial Distributions All Dwarf Galaxies d. Sph and d. E Type Galaxies Globular Clusters – Dwarf Galaxies Connection Workshop

New Data! Color-Magnitude Diagrams SFR (Msun/yr) Star Formation Histories 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 fx. G= fraction of all stars formed in the last x. Gyr Globular Clusters – Dwarf Galaxies Connection Workshop

Star Formation Model 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/pc 2, (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 f 10 G? • General picture is that dwarfs we see today were in larger halos which held more gas and formed more stars early on. • Can improve f 10 G agreement if the dwarfs without early star formation (<1 Gyr) are rejected. • Implies gas accretion is (more) suppressed in low mass halos at early times Globular Clusters – Dwarf Galaxies Connection Workshop

f 10 G

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 f 10 G 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