Neutral Hydrogen Gas in the Circinus Galaxy
Using radio measurements of neutral hydrogen gas (HI) we can not only measure the gas density and HI mass of galaxies but also their velocity. As an example we show the "nearby" Circinus galaxy which lies nearly fully obscured (in visible light) behind the Milky Way. The HI gas distribution (left) shows a complicated spiral density pattern. The velocity field (right) resembles a "spider diagram" from which we measure the galaxy's receding velocity (440 km/s) and its rotational speed (plus/minus 200 km/s) as well as any irregular behaviour (such as a warp). If the large HI envelope of Circinus was visible to our eyes it would appear twice the size of the full moon.
ATCA HI image by B. Koribalski (ATNF, CSIRO), K. Jones, M. Elmouttie (University of Queensland) and R. Haynes (ATNF, CSIRO).