THE COS/UVES ABSORPTION SURVEY OF THE MAGELLANIC STREAM. II. EVIDENCE FOR A COMPLEX ENRICHMENT HISTORY OF THE STREAM FROM THE FAIRALL 9 SIGHTLINE

Abstract

We present a multi-wavelength study of the Magellanic Stream (MS), a massive\ngaseous structure in the Local Group that is believed to represent material\nstripped from the Magellanic Clouds. We use ultraviolet, optical and radio data\nobtained with HST/COS, VLT/UVES, FUSE, GASS, and ATCA to study metal abundances\nand physical conditions in the Stream toward the quasar Fairall 9. Line\nabsorption in the MS from a large number of metal ions and from molecular\nhydrogen is detected in up to seven absorption components, indicating the\npresence of multi-phase gas. From the analysis of unsaturated SII absorption,\nin combination with a detailed photoionization model, we obtain a surprisingly\nhigh alpha abundance in the Stream toward Fairall 9 of [S/H]=-0.30pm0.04 (0.5\nsolar). This value is 5 times higher than what is found along other MS\nsightlines based on similar COS/UVES data sets. In contrast, the measured\nnitrogen abundance is found to be substantially lower ([N/H]=-1.15pm0.06),\nimplying a very low [N/alpha] ratio of -0.85 dex. The substantial differences\nin the chemical composition of MS toward Fairall 9 compared to other sightlines\npoint toward a complex enrichment history of the Stream. We favour a scenario,\nin which the gas toward Fairall 9 was locally enriched with alpha elements by\nmassive stars and then was separated from the Magellanic Clouds before the\ndelayed nitrogen enrichment from intermediate-mass stars could set in. Our\nresults support (but do not require) the idea that there is a metal-enriched\nfilament in the Stream toward Fairall 9 that originates in the LMC.\n