Metallicity and Physical Conditions in the Magellanic Bridge

Abstract

We present a new analysis of the diffuse gas in the Magellanic Bridge (R.A. gtrsim 3^h) based on HST STIS E140M and FUSE spectra of two early-type stars lying within the Bridge and a QSO behind it. We derive the column densities of the H I (from Lyα), N I, O I, Ar I, Si II, S II, and Fe II of the gas in the Bridge. Using the atomic species, we determine the first gas-phase metallicity of the Magellanic Bridge, [Z/H] = - 1.02 +/- 0.07 toward one sight line and -1.7 < [Z/H] < - 0.9 toward the other, a factor of 2 or more smaller than the present-day SMC metallicity. Using the metallicity and N(H I), we show that the Bridge gas along our three lines of sight is ~70%-90% ionized, despite high H I columns, log N(H I) simeq 19.6 - 20.1. Possible sources for the ongoing ionization are certainly the hot stars within the Bridge, hot gas (revealed by O VI absorption), and leaking photons from the SMC and LMC. From the analysis of C II*, we deduce that the overall density of the Bridge must be low (<0.03-0.1 cm^-3). We argue that our findings combined with other recent observational results should motivate new models of the evolution of the SMC-LMC-Galaxy system.

Based on observations made with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer. FUSE is operated for NASA by The Johns Hopkins University under NASA contract NAS5-32985. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.