Globular cluster B023-G78 in the Andromeda galaxy. (Pechetti et al., ArXiv, 2021)

A rare treasure that could shed light on the evolution of black holes has just been found in the closest galactic neighbor to the Milky Way.

In a star cluster in the Andromeda galaxy, also known as M31, astronomers have studied changes in light to identify a black hole that registers almost 100,000 times the mass of the Sun. That puts the beast squarely in the regime “intermediate mass,” both elusive and highly sought after by astronomers for the questions they can answer.

‘In this paper,’ wrote an international team of astronomers led by Renuka Pechetti of Liverpool John Moores University in the UK, ‘we use high-resolution mass models and kinematics to present the detection of an intermediate mass black hole of solar mass (IMBH) of ~ 100,000 with significance greater than 3 sigma.

…work has been published on the prepress server. arXivy accepted for publication by the American Astronomical Society (AAS).

Black holes are very deceptive beasts. Unless they are actively accumulating matter, a process that generates incredibly bright radiation, they do not emit light that we can detect. This makes finding them a matter of detective work, looking at what is happening in your surrounding space.

One of those clues to the presence of a black hole is the orbits of the objects around it.

Most of the black holes that we have detected, using a variety of methods, fall into two mass ranges. There are black holes with stellar mass, up to about 100 times the mass of the Sun; and supermassive black holes, which start in a low range of about a million times the mass of the Sun (and can get incredibly thick from there).

In the middle is a range classified as intermediate, and to say that detections of these black holes are rare is an understatement.

To date, the number of IMBH detections remains incredibly low. This is somewhat annoying; Without intermediate-mass black holes, scientists struggle to figure out how two wildly different mass regimes can coexist.

A robust population of black holes in the intermediate mass range could help us close the gap, offering a mechanism by which stellar-mass black holes can become giants.

This brings us to Andromeda; specifically, a globular star cluster within Andromeda called B023-G078.

B023-G078 is the most massive star cluster of its kind in the galaxy, a roughly spherical star cluster gravitationally bound with a velocity of 6.2 million solar masses.

One way these clusters can form, according to models, is when one galaxy subsumes another. This is a very common phenomenon; the Milky Way has done it several times, as has Andromeda. Globular clusters could be what remains of the galactic nuclei of smaller galaxies that are subsumed by larger ones, with black holes and all.

This is what Pechetti and his colleagues think the origin story for B023-G078 is. They studied the metallic content of the cluster, based on subtle signatures in the light it emits, and determined that it is about 10.5 billion years old, with a metallicity similar to that of other galactic nuclei stripped from the Milky Way.

Then, they studied the way the stars move around the center of the cluster to try to calculate the mass of the black hole that should be there. This yielded a result of around 91,000 solar masses, which makes up about 1.5 percent of the cluster’s mass.

This suggests that the parent galaxy of B023-G078 was a dwarf galaxy, with around 1 billion solar masses. The mass of the Large Magellanic Cloud, a dwarf galaxy orbiting the Milky Way, has been estimated at 188 billion solar masses, and Andromeda is estimated at around 1.5 trillion solar masses.

The team article is available on arXiv,