On the Cosmic Evolution of Fe/Mg in QSO Absorption Line Systems
  We investigate the variation of the ratio of the equivalent widths of the
FeII$\lambda$2600 line to the MgII$\lambda\lambda$2796,2803 doublet as a
function of redshift in a large sample of absorption lines drawn from the
JHU-SDSS Absorption Line Catalog. We find that despite large scatter, the
observed ratio shows a trend where the equivalent width ratio
$\mathcal{R}\equiv W_{\rm FeII}/W_{\rm MgII}$ decreases monotonically with
increasing redshift $z$ over the range $0.55 \le z \le 1.90$. Selecting the
subset of absorbers where the signal-to-noise ratio of the MgII equivalent
width $W_{\rm MgII}$ is $\ge$3 and modeling the equivalent width ratio
distribution as a gaussian, we find that the mean of the gaussian distribution
varies as $\mathcal{R}\propto (-0.045\pm0.005)z$. We discuss various possible
reasons for the trend. A monotonic trend in the Fe/Mg abundance ratio is
predicted by a simple model where the abundances of Mg and Fe in the absorbing
clouds are assumed to be the result of supernova ejecta and where the cosmic
evolution in the SNIa and core-collapse supernova rates is related to the
cosmic star-formation rate. If the trend in $\mathcal{R}$ reflects the
evolution in the abundances, then it is consistent with the predictions of the
simple model.
