C IV Absorption from Galaxies in the Process of Formation

We investigate the heavy element QSO absorption systems caused by gas condensations at high redshift which evolve into galaxies with circular velocity of 100 to 200 km s"-"1 at the present epoch. Artificial QSO spectra were generated for a variety of lines-of-sight through regions of the universe simulated with a hydrodynamics code. The CIV and HI absorption features in these spectra closely resemble observed CIV and HI absorption systems over a wide range in column density if [C/H]#propor to#-3 to -2 is assumed. CIV absorption complexes with multiple-component structure and velocity spreads up to #propor to#600 km s"-"1 are found. The broadest systems are caused by lines-of-sight passing through groups of protogalactic clumps with individual velocity dispersions of less than 150 km s"-"1 aligned along filamentary structures. The largest velocity spreads are kinematic and due to the Hubble expansion between different clumps. Typical clumps have a radius of about 5-10 kpc, a baryonic mass of #propor to#10"9 M_sun and are surrounded by hot gas atmospheres with radius of about 30 kpc. The temperature of most of the gas does not take the photoionization equilibrium value. This invalidates density and size estimates derived from thermal equilibrium models. Consequences for metal abundance determinations are briefly discussed. We predict occasional exceptionally large ratios of CIV to HI column density (up to a third) for lines-of-sight passing through compact halos of hot gas with temperature close to 3.10"5 K. Our model may be able to explain both high-ionization multi-component heavy-element absorbers and damped Ly#alpha# systems ass groups of small protogalactic clumps. (orig.)