A Tour of the Fornax Cluster

Narrator (April Jubett, CXC): Scientists have found evidence that pairs of stars have been kicked out of their host galaxies. This discovery, made using data from NASA's Chandra X-ray Observatory, is one of the clearest examples of stellar pairs being expelled from their galactic base.

Astronomers use the term "binary" system to refer to pairs of stars orbiting around each other. These stellar pairs can consist of combinations of stars like our Sun, or more exotic and denser varieties such as neutron stars or even black holes.

Neutron stars form when a massive star explodes as a supernova and the core of the star collapses onto itself. Under certain conditions, these gargantuan blasts that create the neutron star are not symmetric. The recoil effect can kick the star with such force that it is expelled from the galaxy where it resides. These new Chandra results show that sometimes a companion star is forced to exit the galaxy as well.

How do astronomers look for these banished pairs? If the companion star is close enough, then matter from it will swirl toward the denser neutron star and form a disk around the neutron star. The strong gravitational forces from the neutron star cause the material in this disk to move more rapidly as it approaches the neutron star, and frictional forces in the disk heat the gaseous disk to tens of millions of degrees. At these temperatures, the disk glows in X-ray light.

Astronomers found signatures of X-ray binaries — that is, those binaries that give off X-rays — outside of galaxies in the Fornax galaxy cluster. They did this by studying about 15 days' worth of Chandra data taken between 1999 and 2015. The Fornax cluster is relatively nearby at a distance of some 60 million light years from Earth in the constellation sharing its name.

By combining the large Chandra dataset with optical observations, researchers made a census of X-ray sources within about 600,000 light years of the central galaxy in the Fornax cluster. Astronomers concluded that about 30 sources in the Fornax cluster were likely to be pairs of stars that had been kicked out of the center of their host galaxies. The research team hopes to look for more evicted binaries around other galaxies using Chandra in the future.