Modeling The Star Forming Universe at z=2: Impact of Cold Accretion Flows

TL;DR

This study uses a semi-analytic model including cold accretion and a porosity-based star formation prescription to better match observed properties of star-forming galaxies at redshift 2, highlighting the role of cold flows in galaxy evolution.

Contribution

It introduces a novel semi-analytic model incorporating cold accretion and porosity-based star formation, improving agreement with observed galaxy properties at z=2.

Findings

Cold accretion drives turbulence and star formation efficiency at z=2.

Model reproduces observed high star formation rates and galaxy densities.

Star formation efficiency scales with gas velocity dispersion, linked to cold accretion.

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.