Evolution of the Population of Very Strong MgII Absorbers

We present a study of the evolution of several classes of MgII absorbers, and their corresponding FeII absorption, over a large fraction of cosmic history: 2.3 to 8.7 Gyrs from the Big Bang. Our sample consists of 87 strong (Wr(MgII)>0.3 A) MgII absorbers, with redshifts 0.2<z<2.5, measured in 81 quasar spectra obtained from the Very Large Telescope(VLT)/Ultraviolet and Visual Echelle Spectrograph(UVES) archives of high-resolution spectra (R \sim 45,000). No evolutionary trend in Wr(FeII)/Wr(MgII) is found for moderately strong MgII absorbers (0.3<Wr(MgII)<1.0 A). However, at lower z we find an absence of very strong MgII absorbers (those with Wr(MgII)>1 A) with small ratios of equivalent widths of FeII to MgII. At high z, very strong MgII absorbers with both small and large Wr(FeII)/Wr(MgII) values are present. We compare our findings to a sample of 100 weak MgII absorbers (Wr(MgII)<0.3 A) found in the same quasar spectra by Narayanan et al. (2007). The main effect driving the evolution of very strong MgII systems is the difference between the kinematic profiles at low and high redshifts. At high z, we observe that, among the very strong MgII absorbers, all of the systems with small ratios of Wr(FeII)/Wr(MgII) have relatively large velocity spreads, resulting in less saturated profiles. At low z, such kinematically spread systems are absent, and both FeII and MgII are saturated, leading to Wr(FeII)/Wr(MgII) values that are all close to 1. The high redshift, small Wr(FeII)/Wr(MgII) systems could correspond to sub-DLA systems, many of which have large velocity spreads and are possibly linked to superwinds in star forming galaxies. In addition to the change in saturation due to kinematic evolution, the smaller Wr(FeII)/Wr(MgII) values could be due to a lower abundance of Fe at high z, which would indicate relatively early stages of star formation in those environments.Comment: 20 pages, 14 figures (figure 1 is a set of 87 figures, which is available on the online version), accepted for publication in the MNRA