The sub-damped Ly a system [alpha system] toward HE 0515-4414

Abstract

Context Extragalactic absorbers with column densities exceeding NH I > 2 × 1020 hydrogen atoms cm-2 are generally termed damped Ly alpha (DLA) systems. The absorbers are basically comprised of predominantly neutral gas, which is an essential prerequisite for the formation of stars. The spectroscopy of absorbers offers the best possibility to study the chemical composition of the supposed equivalents of the interstellar medium of local galaxies at an earlier stage of cosmic evolution. Absorbers in the sub-DLA range with NH I > 1019 cm-2 may consist of predominantly neutral gas, if the cosmic UV background is reduced...