An ATCA Survey of HI Absorption in the Magellanic Clouds I: HI Gas Temperature Measurements in the Small Magellanic Cloud

Abstract

We present the first results from the Small Magellanic Cloud portion of a new Australia Telescope Compact Array (ATCA) HI absorption survey of both of the Magellanic Clouds, comprising over 800 hours of observations. Our new HI absorption line data allow us to measure the temperature and fraction of cold neutral gas in a low metallicity environment. We observed 22 separate fields, targeting a total of 55 continuum sources against 37 of which we detected HI absorption; from this we measure a column density weighted mean average spin temperature of $<T_s>=150$ K. Splitting the spectra into individual absorption line features, we estimate the temperatures of different gas components and find an average cold gas temperature of $\sim 30$ K for this sample, lower than the average of $\sim 40$ K in the Milky Way. The HI appears to be evenly distributed throughout the SMC and we detect absorption in $67\%$ of the lines of sight in our sample, including some outside the main body of the galaxy ($N_{\text{HI}}>2\times 10^{21}$ cm${}^{-2}$). The optical depth and temperature of the cold neutral atomic gas shows no strong trend with location spatially or in velocity. Despite the low metallicity environment, we find an average cold gas fraction of $\sim 20\%$, not dissimilar from that of the Milky Way.

arXiv comment22 pages, 16 figures, accepted for publication in ApJS

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