An article accepted for publication in the journal “Astronomy & Astrophysics” reports the observations of an interaction within the Shapley supercluster, one of the largest known structures in the universe, with collisions between galaxies within it. A team of researchers used in particular observations conducted with three radio telescopes: the Australian ASKAP, the South African MeerKat, and the Indian GMRT. That data was complemented with optical frequency observations conducted with the VST in Chile and X-ray observations with NASA’s XMM-Newton space telescope. Galaxy merger processes are occurring among relatively small groups but offer information on consequences at various scales, with influences even on large structures of galaxy clusters.
The Shapley supercluster owes its name to the astronomer Harlow Shapley who, together with various colleagues, began an investigation of galaxies in the southern sky in 1930. Made up of tens of thousands of galaxies, it’s divided into several clusters and smaller groups. All of these structures are bound by gravity and this causes galactic collisions and mergers. For this reason, it’s a very interesting object of study to better understand the evolution of these large cosmic structures.
Radio emissions generated by various galactic collision and merger events were hypothesized and the Shapley supercluster has been of interest to radio astronomers since the 1990s but the limitations of the instruments of the time made it impossible to study these events using radio waves. Today the situation is very different with the Indian GMRT (Giant Metrewave Radio Telescope) radio telescope and with the big SKA project, the next-generation radio telescope, which has already led to the development of precursors such as ASKAP (Australian SKA Pathfinder) in Australia and MeerKat in South Africa.
The combination of three of the world’s most powerful radio telescopes, complemented by ESO’s VLT Survey Telescope (VST) and NASA’s XMM-Newton space telescope, offered a complete picture of what’s happening in the Shapley supercluster. In particular, the radio emissions that generate a sort of bridge between a cluster of galaxies and a group of galaxies within it were detected.
The top image (G. Di Gennaro) shows a composite image of the central area of the Shapley Concentration within the Shapley supercluster. In the background are the emissions at optical frequencies. The emissions of the hot gas present in the area are represented in blue while the radio emissions detected by the MeerKat radio telescope are represented in red.
Tiziana Venturi, director of the Radio Astronomy Institute of the Italian National Institute of Astrophysics, Bologna, first author of the article and Italian head of the Radio Sky 2020 project, commented on the fact that the exceptional emission made it possible to study the regions between clusters of galaxies, which are less dense but ideal environments to look for traces of interaction between these structures. She also talked about the discovery of a couple of other objects: a head-tail radio galaxy and a ram-pressure stripped spiral galaxy probably originated from the collision that generated the emissions.
A head-tail radio galaxy is a galaxy in which strong radio emissions in the nucleus generated by the supermassive black hole at its center are accompanied by an irregular tail of diffuse radio emissions that can span hundreds of thousands of light-years.
A ram-pressure stripped galaxy has suffered the pressure of a fluid, in this case of gas present among the galaxies of its group or cluster. This can have significant effects on the evolution of a galaxy because the gas within it can be stripped from it as it approaches the center of a cluster.
The bottom image (G. Di Gennaro) shows details of the ram-pressure stripped galaxy and the head-tail radio galaxy discovered by the researchers and described in this study. The radio emissions are shown in red superimposed on the optical frequency image.
Clashes between a cluster that contains about 1,000 galaxies and a group that contains 50/100 galaxies is considered minor but its effects also affect large structures within the Shapley supercluster. They’re very long-lasting processes that show the evolution of these cosmic structures. Now it’s possible to observe them thanks to the precursors of the SKA radio telescope, which is scheduled to be activated in a few years to offer even more detailed images of these and other cosmic phenomena.
