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

In a hierarchical cosmogony galaxies build up by continuous merging of smaller structures. 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. The metal and hydrogen absorption features produced by the collapsing structures closely resemble observed QSO absorption systems over a wide range in HI column density. 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 passing through low-density regions far away from fully-collapsed objects. The observed column density ratios of the different ionic species 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 (J ∝ ν−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 an inhomogeneous metal distribution. Observed and simulated Doppler parameter distributions of HI and CIV absorption lines are in good agreement indicating that shock heating due to gravitational collapse is a second important heating agent in addition to photoionization heating. The two-point correlation function of CIV lines agrees reasonably well with the observed correlation function suggesting that its high-velocity tail is caused by occasional alignments of several absorbing clumps still expanding with the Hubble flow. Both high-ionization multi-component heavy-element absorbers and damped Lyman alpha systems can arise from groups of moderately sized protogalactic clumps (Mbaryon∼ 10M⊙). Recent detections of star forming galaxies at similar redshifts are consistent with this picture.