Extraordinary black hole found in neighboring galaxy

The left panel shows a wide-field image of M31 with a red box and an image of B023-G78 where the black hole was found. Ivn der is from the HST.

Astronomers have found a black hole. It is smaller than the black holes we have found at the centers of galaxies, but larger than the black holes that are born when stars explode. One of the only confirmed intermediate-mass black holes is this one.

There are black holes that are 100 times the size of our sun and black holes at the centers of galaxies that are millions of times the size of our sun. "That's a big gap," said senior author and associate professor of astronomy at the University of Utah and co-author of the study. The discovery fills a gap.

The black hole was hidden in a star cluster. The researchers argue that B023-G078 is not a star cluster at all. The outer stars of stripped nuclei were stripped away by the forces of nature. There is a black hole at the center of a dense nucleus that is left behind.

We've found big black holes in stripped nuclei that are much larger than B023-G078. Renuka Pechetti of John Moores University started the research while at the University of Utah and said that they knew there must be smaller black holes in lower mass stripped nuclei. "I think this is a pretty clear case that we have found one of these objects."

The study was published in The Astrophysical Journal.

A decades-long hunch.

A massive star cluster is a spherical collection of stars bound tightly by gravity. The object's overall mass was determined by a single observation. For a long time, he felt it was something else.

I knew that the B023-G078 object was a candidate for a stripped nucleus, and I thought it was one of the most massive objects in Andromeda. We needed data to prove it. We'd been trying to get more observations from various telescopes for a long time. The idea of a black hole within a stripped nucleus was explored when we discovered it.

Pechetti and his team calculated how mass was distributed within the object by modeling its light profile using observational data and images from the Hubble Space Telescope. The signature light profile of a globular cluster is the same as it is in the outer regions. B023-G078 is different. The light at the center is round. The stars have more heavy elements at the center than near the object's edge.

Thelobular star clusters form at the same time. The stripped nuclei can have formation episodes where gas falls into the center of the galaxy and forms stars. The star clusters can be dragged into the center by the forces of the galaxy. It's a dumping ground for a lot of different things. The stars in stripped nuclei will be more complicated. That's what we saw in B023-G078.

The researchers used the object's mass distribution to predict how fast the stars should move at any given location. The stars were moving around the center. The stars at the center were too slow to compare their observations when they built a model without a black hole. They got speeds that matched the data when they added the black hole. There is evidence that the object is a stripped nucleus.

Pechetti said that the stellar velocities show that there is a dark mass at the center. It's hard for clusters to form big black holes. If it's in a stripped nucleus, there must be a black hole left from the smaller galaxy that fell into the bigger one.

The researchers want to observe more stripped nuclei that may hold intermediate mass black holes. There are opportunities to learn more about the black hole population at the centers of low-mass galaxies and how they are built from smaller building blocks.

The stripped nuclei allow us to decipher the details of the past interactions between smaller and larger galaxies.

Sebastian Kamann of the John Moores University is one of the authors.

Renuka Pechetti and her team found a 100,000 M black hole in M31's Most Massive Globular Cluster: A Tidally Stripped Nucleus. There is a book titled "10847/1538-4357/ac339f".

There is information in the Astrophysical Journal.

The extraordinary black hole was found in a neighboring galaxy on January 24.

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