QSO Metal Absorption Systems at High Redshift and the Signature of Hierarchical Galaxy Formation

Abstract

In hierarchical cosmogonies at redshift three the matter content of a typical present-day galaxy is dispersed over several individual clumps embedded in sheet-like structures, often aligned along filaments. We have used hydrodynamical simulations to investigate the spatial distribution and absorption properties of metal enriched gas in such regions of ongoing galaxy formation. Strong CII and SiIV absorption occurs for lines-of-sight passing the densest regions close to the center of the protogalactic clumps, while CIV is a good tracer of the prominent filamentary structures and OVI becomes the strongest absorption feature for lines-of-sight through low-density regions far away from fully-collapsed objects. Observed column density ratios can be well reproduced if a mean metallicity [Z/H] = -2.5, relative abundances as found in metal-poor stars, a UV background with intensity J_{-22} = 3 at the Lyman limit, and either a power law spectrum (alpha = -1.5) or the spectral shape proposed by Haardt & Madau (1996) are assumed. The observed scatter in [C/H] is about a magnitude larger than that in the simulations suggesting inhomogeneous metal distribution. The detailed small and large scale kinematics observed in CIV and HI are also well reproduced.