A MIKE + UVES survey of Sub-Damped Lyman-Alpha Systems at z<1.5
  We have combined the results from our recent observations of Damped and
sub-Damped Lyman-alpha systems with the MIKE and UVES spectrographs on the
Magellan Clay and VLT Kueyen telescopes with ones from the literature to
determine the N(HI)-weighted mean metallicity of these systems based both on
Fe, a depleted element in QSO absorbers and the local ISM, and Zn a relatively
undepleted element. In each case, the N(HI)-weighted mean metallicity is higher
and shows faster evolution in sub-DLAs than the classical DLA systems. Large
grids of photoionisation models over the sub-DLA \nhI range with CLOUDY show
that the ionisation corrections to the abundances are in general small, however
the fraction of ionized H can be up to ~90 per cent. The individual spectra
have been shifted to the rest frame of the absorber and averaged together to
determine the average properties of these systems at z<1.5. We find that the
average abundance pattern of the Sub-DLA systems is similar to the gas in the
halo of the Milky Way, with an offset of ~0.3 dex in the overall metallicity.
Both DLAs and Sub-DLAs show similar characteristics in their relative
abundances patterns, although the DLAs have smaller <[Mn/Zn]> as well as higher
<[Ti/Zn]> and <[Cr/Zn]>. We calculate the contribution of sub-DLAs to the metal
budget of the Universe, and find that the sub-DLA systems at z<1.5 contain a
comoving density of metals Omega_met (3.5-15.8)x10^{5} M_sun Mpc^{-3}, at least
twice the comoving density of metals in the DLA systems. The sub-DLAs do
however track global chemical evolution models much more closely than do the
DLAs, perhaps indicating that they are a less dust biased metallicity indicator
of galaxies at high redshifts than the DLA systems.
