Extraordinary black hole found in neighboring galaxy

Date:
January 24, 2022
Source:
University of Utah
Summary:
At one hundred thousand solar masses, it is smaller than the black
holes we have found at the centers of galaxies, but bigger than
the black holes that are born when stars explode. This makes it
one of the only confirmed intermediate-mass black holes, an object
that has long been sought by astronomers.



FULL STORY ========================================================================== Astronomers discovered a black hole unlike any other. At one hundred
thousand solar masses, it is smaller than the black holes we have found
at the centers of galaxies, but bigger than the black holes that are
born when stars explode.

This makes it one of the only confirmed intermediate-mass black holes,
an object that has long been sought by astronomers.


==========================================================================
"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," said senior author Anil Seth, associate professor of astronomy
at the University of Utah and co- author of the study. "This discovery
fills the gap." The black hole was hidden within B023-G078, an enormous
star cluster in our closest neighboring galaxy Andromeda. Long thought
to be a globular star cluster, the researchers argue that B023-G078 is
instead a stripped nucleus.

Stripped nuclei are remnants of small galaxies that fell into bigger ones
and had their outer stars stripped away by gravitational forces. What's
left behind is a tiny, dense nucleus orbiting the bigger galaxy and at
the center of that nucleus, a black hole.

"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," said lead author Renuka Pechetti of Liverpool John
Moores University, who started the research while at the University of
Utah. "I think this is a pretty clear case that we have finally found
one of these objects." The study published on Jan. 11, 2022, inThe Astrophysical Journal.

A decades-long hunch B023-G078 was known as a massive globular
star cluster -- a spherical collection of stars bound tightly by
gravity. However, there had only been a single observation of the object
that determined its overall mass, about 6.2 million solar masses. For
years, Seth had a feeling it was something else.



==========================================================================
"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, many years and my proposals always
failed," said Seth.

"When we discovered a supermassive black hole within a stripped nucleus
in 2014, the Gemini Observatory gave us the chance to explore the idea."
With their new observational data from the Gemini Observatory and images
from the Hubble Space Telescope, Pechetti, Seth and their team calculated
how mass was distributed within the object by modeling its light
profile. A globular cluster has a signature light profile that has 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 flatter 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.

"Globular star clusters basically 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 gravitational forces of
the galaxy," said Seth. "It's kind of the dumping ground for a bunch of different stuff. So, stars in stripped nuclei will be more complicated
than in globular clusters. And that's what we saw in B023-G078."
The researchers used the object's mass distribution to predict how fast
the stars should be moving at any given location within the cluster
and compared it to their data. 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 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.

"The stellar velocities we are getting gives us direct evidence that
there's some kind of dark mass right at the center," said Pechetti. "It's
very hard 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."
The researchers are hoping to observe more stripped nuclei that may hold
more intermediate mass black holes. These are an opportunity to learn
more about the black hole population at the centers of low-mass galaxies,
and to learn about how galaxies are built up from smaller building blocks.

"We know big galaxies form generally from the merging of smaller galaxies,
but these stripped nuclei allow us to decipher the details of those past interactions," said Seth.

Other authors include Sebastian Kamann of the Liverpool John Moores
University; Nelson Caldwell, Harvard-Smithsonian Center for Astrophysics;
Jay Strader, Michigan State University; Mark den Brok, Leibniz-Institut
fu"r Astrophysik Potsdam; Nora Luetzgendorf, European Space Agency;
Nadine Neumayer, Max Planck Institu"t fu"r Astronomie; and Karina Voggel, Observatoire astronomique de Strasbourg.

========================================================================== Story Source: Materials provided by University_of_Utah. Original written
by Lisa Potter.

Note: Content may be edited for style and length.


========================================================================== Related Multimedia:
* Black_hole_hidden_within_B023-G078,_an_enormous_star_cluster_in_the
Andromeda_galaxy ========================================================================== Journal Reference:
1. Renuka Pechetti, Anil Seth, Sebastian Kamann, Nelson Caldwell, Jay
Strader, Mark den Brok, Nora Luetzgendorf, Nadine Neumayer,
Karina Voggel. Detection of a 100,000 M ⊙ black hole
in M31's Most Massive Globular Cluster: A Tidally Stripped
Nucleus. The Astrophysical Journal, 2022; 924 (2): 48 DOI:
10.3847/1538-4357/ac339f ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/01/220124151048.htm

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