Astronomers discovered a black hole hidden in an enormous star cluster- B023-G078- in our closest neighboring galaxy Andromeda. Unlike any other black hole, this black hole- at one hundred thousand solar masses- is smaller but bigger than those born when stars explode.
This makes the black hole the only confirmed intermediate-mass black hole.
B023-G078 was considered an enormous star cluster. But, it is a stripped nucleus, argue scientists. Stripped nuclei are remnants of small galaxies that fell into bigger ones and stripped their outer stars away by gravitational forces.
Senior author Anil Seth, associate professor of astronomy at the University of Utah and co-author of the study, said, “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, but there aren’t any measurements of black between these. That’s a large gap. This discovery fills the gap.”
Lead author Renuka Pechetti of Liverpool John Moores University, who started the research while at the U, said, “Previously, we’ve found big black holes within massive, stripped nuclei that are much bigger than B023-G078. We knew that there must be smaller black holes in lower mass stripped nuclei, but there’s never been direct evidence. I think this is a pretty clear case that we have finally found one of these objects.”
Seth said, “I knew that the B023-G078 object was one of the most massive objects in Andromeda and thought it could be a candidate for a stripped nucleus. But we needed data to prove it. We’d been applying to various telescopes to get more observations for many years, and my proposals always failed. When we discovered a supermassive black hole within a stripped nucleus in 2014, the Gemini Observatory gave us the chance to explore the idea.”
For this study, the team collected data from Gemini Observatory and images from the Hubble Space Telescope. Using the data, they estimated the mass distribution within B023-G078 by modeling its light profile.
A globular cluster has a signature light profile with the same shape near the center as it does in the outer regions. B023-G078 is different. The light at the center is round and then gets flattened, moving outwards. The chemical makeup of the stars changes too, with more heavy elements in the stars at the center than those near the object’s edge.
Seth said, “Globular star clusters form at the same time. In contrast, these stripped nuclei can have repeated formation episodes, where gas falls into the center of the galaxy and forms stars. And other star clusters can get dragged into the center by the galaxy’s gravitational forces. It’s kind of the dumping ground for a bunch of different stuff. So, stars in stripped nuclei will be more complicated than globular clusters. And that’s what we saw in B023-G078.”
Using the object’s mass distribution, the team predicted how fast the stars should be moving at any given location within the cluster. They then compared it to their data. They found that the highest velocity stars were orbiting around the center.
When they built a model without including a black hole, the stars at the center were too slow compared to their observations. When they added the black hole, they got speeds that matched the data. The black hole adds to the evidence that this object is a stripped nucleus.
Pechetti said, “The stellar velocities we are getting gives us direct evidence that there’s some dark mass right at the center. It’s tough for globular clusters to form big black holes. But if it’s in a stripped nucleus, then there must already be a black hole present, left as a remnant from the smaller galaxy that fell into, the bigger one.”
Seth said, “We know big galaxies generally form from the merging of smaller galaxies, but these stripped nuclei allow us to decipher the details of those past interactions.”
- Renuka Pechetti et al. Detection of a 100,000 M⊙ black hole in M31’s Most Massive Globular Cluster: A Tidally Stripped Nucleus. DOI: 10.3847/1538-4357/ac339f