On the Cosmic Evolution of Fe/Mg in QSO Absorption Line Systems

TL;DR

This study examines how the FeII to MgII equivalent width ratio in quasar absorption systems changes with redshift, revealing a decreasing trend that aligns with models of cosmic chemical evolution driven by supernova activity.

Contribution

It provides observational evidence for the evolution of Fe/Mg abundance ratios over cosmic time and links it to supernova rates and star formation history.

Findings

The FeII/MgII ratio decreases monotonically with redshift.

The mean ratio varies as approximately -0.045 times redshift.

Results are consistent with simple models of chemical evolution involving supernovae.

Abstract

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.