The Magellanic Bridge as a DLA System: Physical Properties of Cold Gas
  toward PKS0312-770
  We measure the physical properties of a local multi-component absorption-line
system at V_sol ~ 200 km/s toward the quasar PKS0312-770 behind the Magellanic
Bridge (MB) using Hubble Space Telescope STIS spectroscopy in conjunction with
photoionization modeling. At an impact parameter of ~ 10 kpc from the Small
Magellanic Cloud (SMC), this sightline provides a unique opportunity to probe
the chemical properties and ionization structure in a nearby absorption line
system with a column density of logN(HI) ~ 20.2, at the transition between
Damped Lyman Alpha (DLA) and sub-DLA systems. We find that metallicity of -1.0
< logZ < -0.5 and ionization parameter of -6 < logU < -5 for three
low-ionization components and logU ~ -2.6 for one high-ionization component.
One component at V_sol = 207 km/s shows an alpha-element abundance log(Si/H) ~
-5.0, making it ~ 0.2 dex more metal rich than both SMC H II regions and stars
within the MB and the SMC. The N/Si ratio in this component is log(N/Si) =
-0.3+/-0.1, making it comparable to other N-poor dwarf galaxies and ~ 0.2 dex
lower than H II regions in the SMC. Another component at V_sol = 236 km/s shows
a similar Si/H ratio but has log(N/Si) = -1.0+/-0.2, indicating a nitrogen
deficiency comparable to that seen in the most N-poor DLA systems. These
differences imply different chemical enrichment histories between components
along the same sightline. Our results suggest that, if these absorbers are
representative some fraction of DLA systems, then 1) DLA systems along single
sight-lines do not necessarily represent the global properties of the absorbing
cloud, and b) the chemical composition within a given DLA cloud may be
inhomogeneous.
