Hubble helps astronomers find rare black hole in Omega Centauri

Located approximately 17,000 light-years away, Omega Centauri has long been known as the brightest and largest globular cluster in the night sky. When observed in areas with low light pollution, it appears nearly as large as a full moon. Globular clusters can contain up to one million stars at the same time and are often found in the outskirts and central regions of galaxies.

However, Omega Centauri features several characteristics that distinguish it from other globular clusters. For example, Omega Centauri rotates much faster than most globular clusters, and its shape is highly flattened. What’s more, Omega Centauri is around 10 times more massive than other large globular clusters, with it being nearly as massive as a small galaxy.

Current estimates have Omega Centauri containing around 10 million stars that are all gravitationally bound to the center of the cluster.

To better understand the motions of the stars within the cluster, Häberle et al. created an enormous catalog of the velocities of around 1.4 million stars using over 500 Hubble images of the cluster. Interestingly, most of the Hubble images used by the team to construct their catalog were not taken for scientific use, but rather to calibrate the telescope’s instrument. Häberle et al.’s catalog is now the largest catalog of motions for any star cluster to date and will be openly available to the public at a later date.

However, when working through their catalog, the team noticed seven stars that were seemingly out of place within Omega Centauri and moving at such incredible velocities that they should have been escaping the cluster—not staying within it.

“We discovered seven stars that should not be there. They were moving so fast that they should escape the cluster and never come back. The most likely explanation is that a very massive object is gravitationally pulling on these stars and keeping them close to the center. The only object that can be so massive is a black hole, with a mass at least 8,200 times that of our Sun,” explained Häberle.

Several studies have been released in the last decade that suggest the presence of an IMBH within Omega Centauri. These studies proposed that a cluster of stellar-mass black holes at the center of Omega Centauri could have contributed to the mass of the IMBH. Additionally, these studies suggested that the lack of fast-moving stars above the velocity needed to escape the cluster’s gravity made the existence of an IMBH less likely.