Cosmic Chemical Evolution

We present some illustrative models of cosmic chemical evolution and compare them with the observed abundances of neutral hydrogen, heavy elements, and dust in damped Lymanα systems. To allow for a wide range of possibilities, we consider closed-box, inflow, and outflow models. The novel feature of our models is a self-consistent correction for the absorbers that are missing from existing samples as a result of the obscuration of background quasars. This is accomplished by assuming that the dust-to-gas ratio is proportional to the mean metallicity in the absorbers. Our models reproduce all of the available data on damped Lymanα systems, including the mean metallicity Z ≍ 0.1 Z_sun at z ≍ 2.2, without any fine tuning of the input parameters. The models are also consistent with the average properties of present-day galaxies. The rapid rise in the mean metallicity with decreasing redshift is caused by high rates of star formation at 1 ≲ z ≲ 2. In contrast, models without obscuration have less star formation at these redshifts and fail to match the mean metallicity in the damped Lymanα systems.