Ly α absorbers at high redshift — large disks or galactic building blocks ?

We investigate the nature of the physical structures giving rise to damped Lyα absorption systems (DLAS) at high redshift. In particular, we examine the suggestion that rapidly rotating large disks are the only viable explanation for the characteristic observed asymmetric profiles of low ionization absorption lines. Using hydrodynamic simulations of galaxy formation in a cosmological context we demonstrate that irregular protogalactic clumps can reproduce the observed velocity width distribution and asymmetries of the absorption profiles equally well. The velocity broadening in the simulated clumps is due to a mixture of rotation, random motions, infall and merging. The observed velocity width correlates with the virial velocity of the dark matter halo of the forming protogalactic clump (∆v ≈ 0.6 vvir for the median values, with a large scatter of order a factor two between different lines-of-sight). The typical virial velocity of the halos required to give rise to the DLAS population is about 100 km s and most standard hierarchical structure formation scenarios can easily account even for the largest observed velocity widths. We conclude that the evidence that DLAS at high redshift are related to large rapidly rotating disks with vcirc ∼> 200 km s −1 is not compelling. Subject headings: galaxies: kinematics and dynamics — galaxies: structure — intergalactic medium — quasars: absorption lines Max-Planck-Institut für Astrophysik, Postfach 1523, 85740 Garching, Germany Steward Observatory, University of Arizona, Tucson, AZ 85721, USA Astronomy Department, California Institute of Technology, Pasadena, CA 91125, USA Hubble Fellow