Holm 15A: Supermassive Black Hole 40 Billion Times the Mass of the Sun Is One of the Biggest Ever Discovered

A supermassive black hole with a mass 40 billion times that of the Sun has potentially been discovered in a galaxy cluster that sits about 700 million light years from Earth. If confirmed, this would be the largest supermassive black hole in the local universe—a region spanning about one billion light years in radius.

Holm 15A is a bright cluster galaxy with an unusually depleted core—its central region appears to be far fainter than any other early-type galaxy that has been modeled in detail, researchers led by Kianusch Mehrgan, from the Max Planck Institute for Extraterrestrial Physics, Germany, explained in a paper posted to the preprint server arxiv.org.

In the study, which has not yet been peer reviewed but has been submitted to The Astrophysical Journal, the team used the European Southern Observatory's Very Large Telescope (VLT)'s to make new observations of Holm 15A's depleted core.

In doing so, they discovered a supermassive black hole with a mass 40 million times that of the Sun. To put that into perspective, Sagittarius A*, the supermassive black hole at the center of the Milky Way, is 4.6 million times the mass of our Sun.

"This is the most massive black hole with a direct dynamical detection in the local universe," the scientists wrote about Holm 15A. They said this is four to nine times bigger than expected given the properties of the galaxy it sits in, and that it could have been produced through a merger of two elliptical galaxies whose cores had already been depleted.

The circumstances that led to the formation of the supermassive black hole in Holm 15A are "probably rare," the researchers said, however it shows black holes of this size can exist.

Professor Andrew Coates, from University College London's Department of Space and Climate Physics, who was not involved in the study, told Newsweek: "This is a remarkable observation of an extremely massive black hole at 40 billion solar masses. This makes it the most massive in our region of the universe, and one of the most massive ever found."

The discovery of the supermassive black hole in Holm 15A follows research published in the Monthly Notices of the Royal Astronomical Society in 2018. In it, researchers used data from NASA's Chandra X-ray to find a group of "ultramassive" black holes in 72 galaxies 3.5 billion light years. They found that half of the black holes identified had a mass ten billion times bigger than our Sun.

"We have discovered black holes that are far larger and way more massive than anticipated," study author Mar Mezcua, from Spain's Institute of Space Sciences, said in a statement. "Are they so big because they had a head start or because certain ideal conditions allowed them to grow more rapidly over billions of years? For the moment, there is no way for us to know."

black hole
Artist impression of a black hole. The supermassive black hole in Holm 15A is estimated to have a mass 40 billion times that of the Sun. iStock