There is a treasure that could shed light on the evolution of black holes.
Astronomers were able to identify a black hole that was 100,000 times the mass of the Sun by studying changes in light. That places the beast squarely in the regime of intermediate mass, which is both elusive and highly sought by astronomy for the questions they can answer.
We have very good detections of the biggest, stellar-mass black holes up to 100 times the size of our Sun, and supermassive black holes at the centers of galaxies that are millions of times the size of our sun.
That is a large gap. This discovery fills a gap.
There is a cluster in the sky. Pechetti et al., The Astrophysical Journal.
Black holes are difficult. They give off no light unless they are actively accreting matter, a process that produces incredibly bright radiation. This makes finding them a lot of detective work.
There is a clue to the presence of a black hole by the objects around it.
The majority of the black holes we have detected fall into two mass ranges. The stellar-mass black holes can be up to 100 times the mass of the Sun, and the supermassive black holes can start at a million times the mass of the Sun.
To say that detections of black holes are rare is an understatement.
The number of detections is incredibly low. Scientists struggle to resolve how two wildly different mass regimes can coexist without intermediate-mass black holes.
A solid population of black holes in the intermediate mass range could help bridge the gap and allow stellar-mass black holes to grow into monsters.
A cluster of stars called B023-G078 is located in Andromeda.
B023-G078 is the most massive star cluster in the universe, with over 6 million stars.
According to models, one way that these clusters can form is when one galaxy subsumes another. The Milky Way and Andromeda have both done it several times. The left of the smaller galaxies that get subsumed by larger ones, black holes and all may be what's left of the globular clusters.
This phenomenon is what the team is led by Renuka Pechetti of John Moores University in the UK.
They studied the metal content of the cluster and found that it has an age of about 10 billion years, which is similar to the age of other stripped galactic cores.
They studied how the stars move around the center of the cluster to calculate the mass of the black hole. 1.5 percent of the mass of the cluster was returned from this result.
This suggests that B023-G078 was a dwarf galaxy, with a solar mass greater than a billion. The Large Magellanic Cloud is a dwarf galaxy that is around the same size as the sun and is estimated to have a solar mass of up to 1.5 trillion.
It is possible that something else accounts for the observations, but none of the alternatives explored fit the data as well as an intermediate-mass black hole.
We knew that there must be smaller black holes in lower mass stripped nuclei, but there has never been direct evidence.
I think this is a pretty clear case that we have found one of these objects.
The findings are reported in a journal.
An earlier version of this article was published in November.