An Aluminum Enhanced Cloud in a CIV Absorber at z = 1.94

Abstract

In the z=1.94 CIV absorption line system in the spectrum of quasar Q1222+228 (z_em=2.04), we find two clouds which have contrasting physical conditions, although they are only at a 17 km/s velocity separation. In the first cloud SiII, SiIV, and CII are detected, and AlII and AlIII column density limits in conjunction with photoionization models allow us to infer that this cloud has a large Si abundance and a small Al abundance relative to a solar abundance pattern. This pattern resembles that of Galactic metal-poor halo stars, which must have formed from such high redshift gas. The second cloud, in contrast, has detected AlII and AlIII (also SiIV and CII), but no detected SiII. We demonstrate, using photoionization models, that Al/Si must be greater than (Al/Si)_Sun in this unusual cloud. Such a ratio is not found in absorption profiles looking through Milky Way gas. It cannot be explained by dust depletion since Al depletes more severely than Si. Comparing to other Al-rich environments, we speculate about the processes and conditions that could give rise to this abundance pattern.