A star cluster in the Andromeda galaxy may be harboring a unique black hole — no longer stellar in size, but not yet supermassive.
Detecting stellar-mass and supermassive black holes
By themselves, black holes do not radiate and remain invisible to telescopes operating in any part of the spectrum. They can be noticed if the hole actively absorbs matter, causing it to glow, or along the orbits of objects rotating close enough. This allows us to estimate the mass of the hole as well.
Observations show that almost all of them are distributed into stellar-mass holes and supermassive ones. The former are formed as a result of supernova explosions and gain tens of solar masses. The latter, located in the active centers of galaxies, reach millions and even billions of solar masses.
What about intermediate black holes?
Not so long ago, the existence of intermediate black holes was predicted, the mass of which lies in the range from 10 to 10 5 masses of the Sun, but it is extremely difficult to detect them, although searches have been going on for a long time.
Candidates for such objects have been found in dwarf galaxies, globular clusters, hyper-powerful X-ray sources, but it is difficult to confirm the presence of black holes in them due to the lack of a generally accepted reliable method for determining their mass.
The mechanism for the formation of black holes of intermediate-mass remains unknown, although it is assumed that they can appear during the merger of black holes of stellar masses.
Perhaps we are bad at detecting them, but it is possible that they are indeed very rare. In any case, each such find attracts attention. Here is what researchers say about such objects:
“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 holes between these,” says astronomer and senior author of the study Anil Seth from the University of Utah.
A new intermediate black hole candidate
A new intermediate-mass black hole candidate is reported in a paper by Renuka Pechetti and colleagues published in The Astrophysical Journal. The object is located in the neighboring galaxy, the Andromeda Nebula (M31), in its largest globular cluster B023-G078.
It includes many thousands of stars with a total mass of 6.2 million solar masses. Based on its content of elements heavier than hydrogen and lithium, astronomers have determined that the age of this cluster is as much as 10.5 billion years.
Judging by the movements of the stars in B023-G078, a black hole with a mass of 91 thousand suns has been preserved in its center, and accounts for about 1.5 percent of the total mass of the cluster.
Scientists suggest that it was once a dwarf galaxy with a mass of about a billion solar masses, which was absorbed by the large Andromeda galaxy, retaining only the central regions with a small black hole. For comparison, Andromeda‘s own mass reaches 1.5 trillion solar masses.
According to researchers, “This discovery fills the gap.”
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• Barrows, R. S., Mezcua, M., & Comerford, J. M. (2019, July 18). A catalog of hyper-luminous X-ray sources and intermediate-mass black hole candidates out to high redshifts. arXiv.org.
• Pechetti, R., Seth, A., & Kamann, S. (2022, January 11). Detection of a 100,000 M⊙ black hole in M31’s Most Massive Globular Cluster: A Tidally Stripped Nucleus. The Astrophysical Journal.
• Potter, L. (2022, January 24). Extraordinary black hole found in neighboring galaxy: @theu.
• ScienceDaily. (2022, January 24). Extraordinary black hole found in neighboring galaxy.
• Starr, M. (n.d.). Astronomers have detected a super rare ‘missing link’ Black Hole in the next galaxy over. ScienceAlert.