MODELING THE STAR-FORMING UNIVERSE AT z=2: IMPACT OF COLD ACCRETION FLOWS

We present results of a semianalytic model (SAM) that includes cold accretion and a porosity-based prescription for star formation. We can recover the puzzling observational results of low V/sigma seen in various massive disk or disk-like galaxies, if we allow 18% of the accretion energy from cold flows to drive turbulence in gaseous disks at z = 2. The increase of gas mass through cold flows is by itself not sufficient to increase the star formation rate sufficiently to recover the number density of (M) over dot(*) > 120 M(circle dot) yr(-1) galaxies in our model. In addition, it is necessary to increase the star formation efficiency. This can be achieved naturally in the porosity model, where star formation efficiency scales alpha sigma, which scales as cloud velocity dispersion. As cold accretion is the main driver for gas velocity dispersion in our model, star formation efficiency parallels cold accretion rates and allows fast conversion into stars. At z similar to 2, we find a space density 10(-4) Mpc(-3) in star-forming galaxies with. (M) over dot(*) > 120 M(circle dot) yr(-1), in better agreement than earlier estimates from SAMs. However, the fundamental relation between. (M) over dot(*) and M(*) is still offset from the observed relation, indicating the need for possibly more efficient star formation at high-z perhaps associated with a role for active galactic nucleus (AGN) triggering.