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

Abstract

We present results of a semi-analytic 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 $\dot{M}_*>120$ M$_{\odot}$ 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 $\propto \sigma$, 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 $z\sim 2$, we find a space density $~10^{-4}$ Mpc$^{-3}$ in star-forming galaxies with $\dot{M}_*>120$ M$_{\odot}$ yr$^{-1}$, in better agreement than earlier estimates form SAMs. However, the fundamental relation between $\dot{M}_*$ 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 AGN triggering.