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1st of October 2019
An ATCA survey of HI absorption in the SMC
by Jameson et al.
Atomic hydrogen gas (HI) is usually the predominant gas in a galaxy. The HI is expected to be split between a cool (~100 K), denser phase and a warm, more tenuous (~6000 K) phase in pressure equilibrium. Observation of the 21-cm HI absorption against background radio continuum sources is the primary way to directly trace the cold and/or optically thick HI in a galaxy. Comparing absorption to nearby HI emission, one can measure the excitation temperature, or spin temperature, of HI gas. Jameson et al. have presented the results from a new Australia Telescope Compact Array (ATCA) HI absorption survey towards 55 sources in the Small Magellanic Cloud. They combined the new ATCA (interferometric) data with archival Parkes (single-dish) data to produce a spectrum of the HI emission for each source, and the ATCA data to search for absorption features.

The plots above show extracted absorption (left) and emission (right) spectra for the source 003824-742212. In the left plot the lines show the absorption spectrum and shaded areas the 1-sigma uncertainty level. Gray represents the highest spectral resolution of 0.2 km/s, blue represents the data smoothed to 0.6 km/s. In the right plot, the black line shows the extracted emission spectrum, and the filled light gray area shows the estimated uncertainty. The dashed gray line shows the emission spectrum averaged over the primary beam size of individual ATCA dish (34 arcmin). The HI appears to be evenly distributed throughout the SMC and absorption was detected in 67% of the lines of sight in the sample, including some outside the main body of the galaxy. The optical depth and temperature of the cold neutral atomic gas shows no strong trend with location spatially or in velocity. More information is give in the the data in the paper to be published in the Astrophysical Journal Supplement Series.

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