NASA finds “unprecedented” black hole cluster near Andromeda’s central bulge

NASA has discovered an unprecedentedly large cluster of black holes in our nearest galactic neighbor, Andromeda.

The 26 black hole candidates were spotted with the Chandra X-ray Observatory from more than 150 observations spread over 13 years.

Each of the black holes is of the kind that forms after a star collapses in on itself. As they suck in material from other stars that orbit or pass nearby, they also suck in material that gives out X-rays as it is consumed. It's this that Chandra spotted.

To filter Andromeda's black holes from other X-ray sources—such as neutron stars, or much larger black holes much farther away than Andromeda—the astrophysicists had to look for attributes like brightness, variability and color.

These black holes are more easily spotted than most because they have companion stars to provide the material that emits X-rays. "While we are excited to find so many black holes in Andromeda, we think it's just the tip of the iceberg," said Robin Barnard of the Harvard-Smithsonian Centre for Astrophysics, lead author of the study published in The Astrophysical Journal. "Most black holes won't have close companions and will be invisible to us."

The 26 black holes join a previous group of nine found using Chandra data. They are close to Andromeda's "central bulge," the spherical network of huge, old stars that is at the heart of most galaxies.

Andromeda's bulge is larger than the Milky Way's, and that larger number of stars means there is also a corresponding greater number of black holes for us to spot.