How do galaxies get their gas?

Not the way one might have thought. In hydrodynamic simulations of galaxy formation, some gas follows the traditionally envisioned route, shock heating to the halo virial temperature before cooling to the much lower temperature of the neutral ISM. But most gas enters galaxies without ever heating close to the virial temperature, gaining thermal energy from weak shocks and adiabatic compression, and radiating it just as quickly. This “cold mode” accretion is channeled along filaments, while the conventional, “hot mode” accretion is quasi-spherical. Cold mode accretion dominates high redshift growth by a substantial factor, while at z < 1 the overall accretion rate declines and hot mode accretion has greater relative importance. The decline of the cosmic star formation rate at low z is driven largely by geometry, as the typical cross section of filaments begins to exceed that of the galaxies at their intersections. The conventional sketch of galaxy formation has its roots in classic papers of the late ’70s and early ’80s, with the initial discussions of collapse and cooling criteria by Binney (1977), Rees & Ostriker (1977), and Silk (1977), the addition of dark matter halos by White & Rees (1978), and the disk formation model of Fall & Efstathiou (1980). According to this sketch, gas falling into a dark matter potential well is shock heated to approximately the halo virial temperature, Tvir = 10 (vcirc/167 km s ) K. Gas in the dense, inner regions of this shock heated halo radiates its thermal energy, settles into a centrifugally supported disk, and forms stars. Mergers of disks can scatter stars onto disordered orbits, producing spheroidal systems, which may regrow disks if they experience sub-