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

## TL;DR

This study uses ATCA HI absorption data to measure cold gas temperatures and fractions in the low metallicity environment of the Small Magellanic Cloud, revealing similarities to the Milky Way despite environmental differences.

## Contribution

First detailed HI absorption survey of the SMC providing measurements of cold gas temperature and fraction in a low metallicity environment.

## Key findings

- Mean spin temperature of 150 K in the SMC.
- Average cold gas temperature of ~30 K, lower than in the Milky Way.
- Cold gas fraction of ~20%, comparable to the Milky Way.

## 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.