The Subclasses of Metal-Rich Absorbers

Abstract

We have derived the evolution of several physical parameters of metal-rich absorbers from a comparison of the evolution in redshift of the different subclasses of systems. At z > 2, absorption from singly-ionized elements always arises from regions of large opacity to UV ionizing photons. This is no longer the case at z < 1, and singly-ionized elements are then also found in optically thin regions. Consequently, there is a decrease in the ionization level of absorbers with decreasing redshift. We find higher heavy element abundances at z < 1 than at z > 2. In the assumption of photoionization by the UV metagalactic field, analysis of the observed column densities for different atoms and ions implies a decrease in the total column density, thus mass, of Mg II clouds between z = 2 and 0.5. There is significant clustering of both C IV and Mg II redshifts up to 600 km s⁻¹, with a strong peak at the lowest velocity separations observed of ~30 km s⁻¹. The rest-frame equivalent width of C IV and Mg II scales linearly with the number of subcomponents and is correlated with the total velocity separation between the subcomponents. The evolution of strong Mg II and C IV redshifts thus reflects an evolution in the clustering properties of the absorbers. The high ionization O VI absorption, detected at intermediate and high redshifts, originates from a homogeneous gaseous region of low density in which C II–C III clouds are embedded. The spread in observed ionization states favors photoionization for both phases, and C IV absorption should arise from both regions. Chemical evolution coupled to fragmentation of galactic halos in the early stages of galaxy formation followed by infall of halo gas onto galactic disks can explain most of the observed evolutions.