How Do Galaxies Get Their Gas?

We examine the temperature history of gas accreted by forming galaxies in SPH simulations. About half the gas shock heats to roughly the virial temperature of the galaxy potential well before cooling, condensing, and forming stars, but the other half radiates its acquired gravitational energy at much lower temperatures, typically T<10^5 K, and the histogram of maximum gas temperatures is clearly bimodal. The "cold mode" of gas accretion dominates for low mass galaxies (M_baryon < 10^{10.3}Msun or M_halo < 10^{11.4}Msun), while the conventional "hot mode" dominates the growth of high mass systems. Cold accretion is often directed along filaments, allowing galaxies to efficiently draw gas from large distances, while hot accretion is quasi-spherical. The galaxy and halo mass dependence leads to redshift and environment dependence of cold and hot accretion rates, with cold mode dominating at high redshift and in low density regions today, and hot mode dominating in group and cluster environments at low redshift. Star formation rates closely track accretion rates, and we discuss the physics behind the observed environment and redshift dependence of galactic scale star formation. If we allowed hot accretion to be suppressed by conduction or AGN feedback, then the simulation predictions would change in interesting ways, perhaps resolving conflicts with the colors of ellipticals and the cutoff of the galaxy luminosity function. The transition between cold and hot accretion at M_h ~ 10^{11.4}Msun is similar to that found by Birnboim & Dekel (2003) using 1-d simulations and analytic arguments. The corresponding baryonic mass is tantalizingly close to the scale at which Kauffmann et al. (2003) find a marked shift in galaxy properties. We speculate on connections between these theoretical and observational transitions.