A rare treasure that could shed light on the evolution of black holes has been discovered in the Milky Way’s nearest large galactic neighbor.
In a cluster of stars in the Andromeda galaxy, also known as M31, astronomers have studied light changes to identify a black hole with a mass nearly 100,000 times that of the Sun. This puts the beast squarely in “intermediate mass” mode – both elusive and highly sought after by astronomers for the questions they can answer.
“We have very good results in detecting the largest stellar-mass black holes, which are 100 times the size of our Sun, and supermassive black holes at the centers of galaxies, which are millions of times larger than our Sun. any measurements of black holes between them,” says astronomer and senior study author Anil Seth of the University of Utah.
“This is a big gap. This discovery fills that gap.”
Black holes are very cunning beasts. Unless they’re actively accreting matter, a process that generates incredibly bright radiation, they don’t emit light that we can detect. This makes finding them the subject of detective work, looking at what is going on in the space around them.
One such clue to the presence of a black hole is the orbit of objects around them.
Most of the black holes we have detected using various methods fall into two mass ranges. There are black holes of stellar mass, about 100 times the mass of the Sun; and supermassive black holes, which start at a low distance of about a million times the mass of the Sun (and can become incredibly choppy from there).
In the middle is the range classified as intermediate, and to say that the detection of these black holes is rare is to say nothing.
To date, IMBH detections remain incredibly low. It’s kind of an annoyance; without intermediate-mass black holes, scientists struggle to work out how two vastly different mass regimes can coexist.
A dense population of black holes in the medium-mass range could help us bridge the gap by suggesting a mechanism by which stellar-mass black holes can grow into giants.
This brings us to Andromeda; in particular, a globular cluster of stars in Andromeda called B023-G078.
B023-G078 is the most massive such star cluster in the galaxy, a nearly spherical, gravitationally bound cluster of stars with a mass of 6.2 million solar masses.
According to the models, one way these clusters form is when one galaxy swallows another. This is a very common occurrence; The Milky Way has done this several times, as has Andromeda. Globular clusters could be what’s left of the galactic nuclei of smaller galaxies being swallowed up by larger ones, black holes and everything else.
This phenomenon is what a team led by first author and astronomer Renuka Pechetti of Liverpool’s John Moores University in the UK believes is the origin story of B023-G078.
They studied the cluster’s metal content based on subtle signatures in the light it emits, and determined that it is about 10.5 billion years old and has a metallicity similar to that of other galactic-depleted cores in the Milky Way.
They then studied how stars move around the center of the cluster to try and calculate the mass of the black hole that should be there. This gave a result of about 91,000 solar masses, which is about 1.5 percent of the mass of the cluster.
This suggests that the host galaxy B023-G078 was a dwarf galaxy with a mass of about a billion solar masses. The mass of the Large Magellanic Cloud – a dwarf galaxy orbiting the Milky Way – is estimated at 188 billion solar masses, while the mass of Andromeda is estimated at about 1.5 trillion solar masses.
Perhaps the observations are explained by something else, but none of the alternatives the team explored fit the data as well as an intermediate-mass black hole.
“We knew that there should be smaller black holes in lower-mass nuclei, but there was never any direct evidence,” Pecetti says.
“I think it’s a pretty obvious case that we finally found one of these objects.”
The results are reported in Astrophysical journal.
The previous version of this article was published in November 2021.