Metal Abundances in the Magellanic Stream

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Abstract
We report on the first metallicity determination for gas in the Magellanic Stream, using archival Hubble Space Telescope (HST) GHRS data for the background targets Fairall 9, III Zw 2, and NGC 7469. For Fairall 9, using two subsequent HST revisits and new Parkes multibeam narrowband observations, we have unequivocally detected the MS I H I component of the Stream (near its head) in S II λλ1250, 1253, yielding a metallicity of [S II/H] = -0.55 ± 0.06(r)(s), where r and s are the associated random and systematic uncertainties, consistent with either an SMC or LMC origin and with the earlier upper limit set recently by Lu, Savage, & Sembach. We also detect the saturated Si II λ1260 line, but set a lower limit of only [Si II/H] -1.5. We present two HST serendipitous detections of the Stream, seen in Mg II λλ2796, 2803 absorption with column densities of (0.5–1) × 1013 cm-2 toward the Seyfert galaxies III Zw 2 and NGC 7469. These latter sight lines probe gas near the tip of the Stream (~15° from the peak of the MS V H I component and ~80° down-Stream of Fairall 9). For III Zw 2, the lack of an accurate H I column density determination and the uncertain Mg III ionization correction severely limit the degree to which we can constrain [Mg/H]; we found a lower limit of [Mg II/H I] -1.3 for this sight line. For NGC 7469, an accurate H I column density determination exists, but the extant Faint Object Spectrograph (FOS) spectrum limits our ability to constrain the Mg II column density, and we conclude that [Mg II/H I] -1.5 for this sight line. Ionization corrections associated with Mg III and H II suggest that the corresponding [Mg/H] may range lower by ~0.3–1.0 dex. However, an upward revision of ~0.5–1.0 dex would be expected under the assumption that the Stream exhibits a dust depletion pattern similar to that seen in both the Large and Small Magellanic Clouds. While our abundance analysis allows us to rule out a primordial origin for the Stream, the remaining systematic uncertainties in the H I column density along the lines of sight makes it difficult to differentiate between an LMC and an SMC origin.
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