Ring galaxies are extremely rare examples of what head-on collisions between galaxies can produce. During these events, a galaxy's spiral structure is lost and replaced by a wide ring formed around a central galactic bulge. Researchers have used data collected by NASA’s Chandra X-Ray Observatory to discover that these rings are populated by extreme objects like black holes and neutron stars.
Publishing their findings in the Astrophysical Journal, the team investigated seven ring galaxies. Chandra revealed 63 X-ray sources present in the rings of these galaxies, and 50 of them were classified as ultraluminous. These objects produced between hundreds and thousands of times more X-rays than regular binary systems that host black holes or neutron stars.
The researchers found an unexpectedly high number of these extreme sources, which has important implications for various astronomical fields. These objects have only recently experienced the collision events that formed them. Well, recently in cosmic terms. Probably just a few hundred million years ago.
Clearly, some of the secrets to the formation of these extreme objects can be found in the collisions that produce ring galaxies. The interaction leads to the formation of the ring, where gas is compressed into stars by the gravitational interaction of the merger. It localizes a brand-new population of stars within the ring, making it appear bright and blue. Some of those new-born stars are quite massive and end up going supernova just a few million years later.
The supernovae create neutron stars and black holes, some of which become the X-ray sources detected in the study. At the moment, researchers don’t know the exact nature of the ultraluminous sources in these galaxies. Both stellar-sized black holes and neutron stars can produce intense X-ray emissions if they are surrounded by enough material to funnel along their strong magnetic field lines. Some of these objects might have companions from which the material is stolen.
Another potential interpretation is that some of these objects are even bigger black holes, like the ones discovered by gravitational wave observatories over the last few years. To explain Chandra's dramatic observations, these black holes would have to ingest large quantities of material.
The researchers hope to investigate the nature of these objects in greater detail and find out why ring galaxies are particularly good at making them.