Element Abundances at High-redshift: Magellan MIKE Observations of
  sub-Damped Lyman-alpha Absorbers at 1.7 < z <2.4
  We present chemical abundance measurements from high-resolution observations
of 5 sub-damped Lyman-alpha absorbers at 1.7 < z < 2.4 observed with the
Magellan Inamori Kyocera Echelle (MIKE) spectrograph on the 6.5-m Magellan II
Clay telescope. Lines of Zn II, Mg I, Mg II, Al II, Al III, S II, Si II, Si IV,
C II, C II*, C IV, Ni II, Mn II and Fe II were detected and column densities
were determined. The metallicity of the absorbing gas, inferred from the nearly
undepleted element Zn, is in the range of < -0.95 to +0.25 dex for the five
absorbers in our sample, with three of the systems being near-solar or
super-solar. We also investigate the effect of ionisation on the observed
abundances using photoionisation modelling. Combining our data with other
sub-DLA and DLA data from the literature, we report the most complete existing
determination of the metallicity vs. redshift relation for sub-DLAs and DLAs.
We confirm the suggestion from previous investigations that sub-DLAs are, on
average, more metal-rich than DLAs and evolve faster. We also discuss relative
abundances and abundance ratios in these absorbers. The more metal-rich systems
show significant dust depletion levels, as suggested by the ratios [Zn/Cr] and
[Zn/Fe]. For the majority of the systems in our sample, the [Mn/Fe] vs. [Zn/H]
trend is consistent with that seen previously for lower-redshift sub-DLAs. We
also measure the velocity width values for the sub-DLAs in our sample from
unsaturated absorption lines of Fe II 2344, 2374, 2600 A, and examine where
these systems lie in a plot of metallicity vs. velocity dispersion. Finally, we
examine cooling rate vs. H I column density in these sub-DLAs, and compare this
with the data from DLAs and the Milky Way ISM. We find that most of the systems
in our sample show higher cooling rate values compared to those seen in the
DLAs.
