An article submitted for publication in “The Astrophysical Journal” reports the discovery of an ultramassive black hole at the center of the galaxy Holmberg 15A, a supergiant elliptical galaxy that is the dominant central member of the Abell 85 galaxy cluster. A team of astronomers led by Kianusch Mehrgan of the Max Planck Institute for Extraterrestrial Physics, Garching bei München, Germany, used the MUSE instrument mounted on ESO’s VLT in Chile to study that galaxy estimating that the mass of that black hole is about 40 billion times the Sun’s.
About 700 million light years from Earth, the galaxy Holmberg 15A – whose name is sometimes shortened to Holm 15A – was discovered only in 1937 despite being much larger and more massive than the Milky Way. It’s the dominant central galaxy of the Abell 85 galaxy cluster, which includes more than 500 galaxies. In short, it’s all really huge, and the black hole at the center of that galaxy is gigantic as well.
Astronomers assume that galaxies have at their center a supermassive black hole even though the definition is a bit vague as research in recent years is exploring the existence of intermediate-mass black holes that make it difficult to provide minimum mass limits for the higher class. Regarding the higher limits, astronomers created a further class of objects that are extreme even for those standards, ultramassive black holes, with masses greater than 10 billion times the Sun’s.
So far very few objects that can fall into the category of ultramassive black holes have been identified with plausible estimates of their mass. In the case of the one at the center of the galaxy Holmberg 15A there was already the strong suspicion that it fell into the category but the estimates of its mass made in the past years provided very different figures, even if all very high. This new research could provide some more precise answers.
In this case, the researchers used the Multi-Unit Spectroscopic Explorer (MUSE) installed on the Very Large Telescope (VLT) to examine the galaxy Holmberg 15A. On November 16, 2017 and August 10, 2018 MUSE provided wide-field spectroscopic data from this galaxy which, in very simple words, allowed to track the movements of the stars around it and consequently to deduce its gravitational attraction and its mass, which turned out to be about 40 billion times the Sun’s. It’s a mass about 10,000 times greater than that of the supermassive black hole at the center of the Milky Way.
According to the researchers, that ultramassive black hole probably formed after the merger of two galaxies and their respective supermassive black holes. To test this theory and get more information they intend to continue studying it. If their estimate is confirmed, it’s one of the biggest known black holes, a further reason of interest to better understand these extreme objects and to test the laws of physics.