SKA pathfinders reveal radio footprints of a minor merger

BY DR DARIA GUIDETTI (INAF)

An international group of researchers led by the Italian National Institute of Astrophysics (INAF) has revealed a weak “bridge” of radio emission connecting a galaxy cluster to a galaxy group, within the Shapley Supercluster, interpreted as the echo of a collision between the two.

The results of this study, part of the Radio Sky 2020 cooperation*, is about to be published in the Astronomy & Astrophysics journal. Observations were carried out with three SKA pathfinders: ASKAP in Australia, MeerKAT in SouthAfrica, and the Giant Metrewave Radio Telescope (GMRT) in India. Their observations were combined with optical data from ESO’s VLT Survey Telescope and X-ray data from ESA’s XMMNewton space telescope.

Composite image of the central region of the Shapley Supercluster. In the background, the optical emission of the galaxies seen by ESO-VST in r-band; in blue, the hot gas emission permeating the whole region detected by XMM-Newton; in red, the radio emission detected by MeerKAT.

Credit: G. Di Gennaro

With more than 8,000 galaxies, the Shapley Supercluster is the largest known gravitationally-bound structure. It is at about 650 million light years from us in the southern Centaurus constellation and is a site of formation of large structures at the present cosmological epoch. It has been in the crosshairs of radio telescopes for 30 years, but the lack of interferometers with the adequate sensitivity in the Southern Hemisphere prevented the detection of the weak diffuse radio emission such as that discovered in this study.

“For a long time we had a study of this type in mind,” says Dr Tiziana Venturi, Director of INAF’s Radio Astronomy Institute and lead author of the article. “Finally, ASKAP and MeerKAT have allowed us to access the Shapley Supercluster with the appropriate radio resolution and sensitivity, which, in synergy with other X-ray and optical data belonging to the team, allowed a very detailed study.”

The detected radio emission is interpreted as a result of a collision between a cluster and a group of galaxies, an example of the so-called minor mergers, events of mass assembly involving lower energy than collisions between galaxy clusters of similar size and/or larger mass (major mergers), but much more common in the Universe.

“Until now it was not clear whether scale relations between the various observational quantities would also apply to these low energy mergers,” continues Dr Venturi, “and our results, in addition to confirming the existence of the radio emission we had predicted, show that even minor mergers can affect both individual galaxies and entire structures of clusters and groups of galaxies, leaving detectable traces at all scales.”

The study also reports the discovery of a very peculiar head-tail radio galaxy and a ram-pressure stripped spiral galaxy, whose origin is traced back to the same minor merger event. The radio data exploited in this study represents the state of the art of the SKA-project precursors and pathfinders, and provide only a first taste of the wealth of information and discoveries that SKA radio telescopes will offer on the weak population of cluster radio sources.

*Radio Sky 2020 is a bilateral scientific and technological cooperation project between Italy and South Africa funded by the Italian Ministry of Foreign Affairs and International Cooperation and the National Research Foundation of South Africa.