An ATCA Survey of H I absorption in the Magellanic Clouds. I. H I gas temperature measurements in the Small Magellanic Cloud

We present the first results from the Small Magellanic Cloud portion of a new Australia Telescope Compact Array H I absorption survey of both of the Magellanic Clouds, comprising over 800 hr of observations. Our new H I 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 H I 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 ∼ 30 K for this sample, lower than the average of ∼40 K in the Milky Way. The H I 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_{H I} > 2 x 10²¹ cm^-2). The optical depth and temperature of the cold neutral atomic gas show no strong trend with location spatially or in velocity. Despite the low-metallicity environment, we find an average cold gas fraction of ∼20%, not dissimilar from that of the Milky Way.