Astronomers discovered a black hole concealed inside a huge star cluster, B023-G078, in the Andromeda galaxy, our nearest neighboring galaxy.
With a mass of one hundred thousand solar masses, this black hole is smaller than any other black hole yet larger than those developed when stars explode.
This makes the black hole the only verified black hole with an intermediate-mass.
B023-G078 was regarded as a massive star cluster. However, experts contend that the nucleus is empty. The nucleus of stripped galaxies is the relics of smaller galaxies that collided with larger ones and had their surrounding stars stripped 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 centres 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.”
The researchers acquired data from the Gemini Observatory and photos from the Hubble Space Telescope for their investigation. Using the data, they modeled the light profile of B023-G078 to determine its mass distribution.
Near the core of a globular cluster, the characteristic light profile has the same form as the outside areas. B023-G078 is different. The light in the center is spherical and becomes flattered as it moves outward. The chemical composition of the stars also varies, with the central stars containing more heavy elements than those towards the object's periphery.
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 centre of the galaxy and forms stars. And other star clusters can get dragged into the centre 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 scientists projected the speed at which stars inside the cluster should be travelling at any given position. They then matched it to their own data. They discovered that the fastest-moving stars orbited the center.
When scientists constructed a model without a black hole, the central stars moved too slowly relative to their observations. When the black hole was introduced, the speeds matched the data. The black hole adds to the evidence that this item is a nucleus that has been stripped.
Pechetti said, “The stellar velocities we are getting gives us direct evidence that there’s some dark mass right at the centre. 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.”
Reference(s): Peer-Reviewed Research
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