“The Dish” detects star fuel reservoirs around radio monsters

The HI Parkes All-Sky Survey (HIPASS) was completed over a decade ago and still represents the most comprehensive census of the atomic hydrogen (HI) content of our local Universe. The 21-cm signal of hydrogen can be picked up by radio telescopes with relative ease from local galaxies and is used to map out the speed at which they are spinning and to trace the fuel available for future star formation.

In their current paper, our University of Sydney based researchers Dr James Allison and Prof Elaine Sadler, together with their previous third year project student Alex Meekin, have used HIPASS data to search for cold clumps of hydrogen in galaxies that host the most radio-bright supermassive black holes. These clouds of hydrogen pass in front of the radio-emitting plasma and absorb some of the radiation, manifesting as a silhouette in the radio signal.

Using a robust technique developed by the FLASH (First Large Absorption Survey in HI) team at the University of Sydney, they carried out a survey of over two hundred radio galaxies. In four of these, they detected hydrogen absorption, all of which are amongst the most radio bright and which are known to have lots of cold material in the form of dust and molecular gas.

In one galaxy, hydrogen absorption was found for the first time – its silhouette in the radio signal indicating that the hydrogen is located in an obscuring disc that is of similar orientation to the edge-on disc of stars seen in the optical images. As has been noted in previous such surveys, these detections seem to be dominated by those radio sources that are small and powerful, concentrating their radio emission behind the hydrogen gas.

Importantly, these results indicate that imminent early-science surveys with the Australian Square Kilometre Array Pathfinder (ASKAP) will be able to detect strong hydrogen absorption in such galaxies at much earlier times in the history of the Universe.

by J. Allison

Publication details:

J. R. Allison, E. M. Sadler, A. M. Meekin in MNRAS (2014) “A search for associated HI absorption in nearby galaxies using HIPASS”