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.
