Making damped Lyman-α systems in semi-analytic models

The velocity profiles of weak metal absorption lines can be used to observationally probe the kinematic state of gas in damped Lyman-α systems. Prochaska and Wolfe [5] have argued that the flat distribution of velocity widths (Δv) combined with the asymmetric line profiles indicate that the DLAS are disks with large rotation velocities (∼200 km/s). An alternative explanation has been proposed by Haehnelt, Steinmetz, and Rauch (HSR) [2], in which the observed large velocity widths and asymmetric profiles can be produced by lines of sight passing through two or more clumps each having relatively small internal velocity dispersions. We investigate the plausibility of this scenario in the context of semi-analytic models based on hierarchical merging trees and including simple treatments of gas dynamics, star formation, supernova feedback, and chemical evolution. We find that all the observed properties of the metal-line systems including the distribution of Δv and the asymmetric profiles, can be reproduced by...