High-z MgII absorption QSOs with FIRE. II.

Using new measurements of rest-frame UV lines from optical spectra of the same targets, we compare the high-redshift sample with carefully constructed low-redshift control samples from the literature to study evolutionary trends from z=0-->5.33 (>12Gyr). We observe a significant strengthening in the characteristic N(HI) for fixed MgII equivalent width as one moves toward higher redshift.

Indeed, at our sample's mean <Z>=3.402, all MgII systems are either damped Ly{alpha} absorbers (DLAs) or sub-DLAs, with 40.7% of systems exceeding the DLA threshold (compared to 16.7% at <Z>=0.927). We set lower limits on the metallicity of the MgII systems where we can measure HI; these results are consistent with the full DLA population. The classical MgII systems (W({lambda}2796)_0_=0.3-1.0{AA}), which preferentially associate with sub-DLAs, are quite metal rich at ~0.1 solar.