THE MAGELLANIC BRIDGE AS A DAMPED LYMAN ALPHA SYSTEM: PHYSICAL PROPERTIES OF COLD GAS TOWARD PKS 0312-770*

摘要

We measure the physical properties of a local multicomponent absorption-line system at V ☉ ~200 km s–1 toward the quasar PKS 0312-770 behind the Magellanic Bridge (MB) using Hubble Space Telescope Space Telescope Imaging Spectrograph (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 ~20.2, at the transition between damped Lyα (DLA) and sub-DLA systems. We find that metallicity of –1.0 log(Z/Z ☉) –0.5 and ionization parameter of –6 log U –5 for three low-ionization components and log U ~ –2.6 for one high-ionization component. One component at V ☉ = 207 km s–1 shows an α-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 ☉ = 236 km s–1 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 sightlines do not necessarily represent the global properties of the absorbing cloud, and (2) the chemical composition within a given DLA cloud may be inhomogeneous.