Star formation in damped Ly$\alpha$ systems

Abstract

We determine star formation rates for a sample of 50 DLAs from properties of the absorbing gas alone. Assuming thermal balance, we determine the grain photoelectric heating rate of the neutral gas from the [C II] $158\,\mu$ m cooling rate per H atom, $\ell_{c}$, inferred from C II$^{*}$ 1335.7 and damped Ly$\alpha$ absorption lines. We deduce the star formation rate per unit area and the FUV luminosity per unit co-moving volume, ${\cal L}_{\nu}^{DLA}$. Comparison of ${\cal L}_{\nu}^{DLA}$ with the luminosity density of Lyman Break Galaxies, ${\cal L}_{\nu}^{LBG}$, shows that ${\cal L}_{\nu}^{DLA}>>{\cal L}_{\nu}^{LBG}$ for most models. These models are ruled out if our assumption that LBGs dominate the total FUV luminosity density of the Universe is correct. The only feasible models are those in which ${\cal L}_{\nu}^{DLA}\approx {\cal L}_{\nu}^{LBG}$. We conclude that DLAs in which $\ell_{c}>10^{-27.1}$ ergs s$^{-1}$H$^{-1}$ contain centrally located LBGs, while the gas in DLAs with $\ell_{c}<10^{-27.1}$ ergs s$^{-1}$H$^{-1}$ is heated by background radiation alone.