Making damped Lyman-α systems in semi-analytic models

The velocity profiles of weak metal absorption lines can be used toobservationally probe the kinematic state of gas in damped Lyman-alpha systems.Prochaska and Wolfe have argued that the flat distribution of velocity widthscombined with the asymmetric line profiles indicate that the DLAS are diskswith large rotation velocities. An alternative explanation has been proposed byHaehnelt, Steinmetz, and Rauch, in which the observed large velocity widths andasymmetric profiles can be produced by lines of sight passing through two ormore clumps each having relatively small internal velocity dispersions. Weinvestigate the plausibility of this scenario in the context of semi-analyticmodels based on hierarchical merging trees and including simple treatments ofgas dynamics, star formation, supernova feedback, and chemical evolution. Wefind that all the observed properties of the metal-line systems including thedistribution of velocities and the asymmetric profiles, can be reproduced bylines of sight passing through sub-clumps that are bound within largervirialized dark matter halos. In order to produce enough multiple hits, we findthat the cold gas must be considerably more extended than the optical radius ofthe proto-galaxies, perhaps even beyond the tidal radius of the sub-halo. Thiscould occur due to tidal stripping or supernova-driven outflows.Comment: 4 pages, 2 figures, uses aipproc.sty, to appear in After the Dark Ages: The 9th Annual October Astrophysics Conferenc