  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.
