Extraordinary black hole found in neighboring galaxy

Astronomers have discovered a black hole with 100,000 solar masses that was hiding in the Andromeda galaxy, inside a large cluster of stars: it is a strange case among the so-called intermediate-mass black holes.

An international group of astronomers has discovered a black hole like no other. At 100,000 solar masses, it is smaller than black holes that have been found in the centers of galaxies, but larger than black holes that are born when stars explode and create a supernova. This makes it one of the few confirmed intermediate-mass black holes: it was hidden inside B023-G078, a huge star cluster in our nearest neighbor galaxy Andromeda.

The Andromeda Galaxy, also known as Messier 31, has a diameter of 220 thousand light years if its galactic halo is considered and about 150 thousand light years between the ends of its arms. In addition to being the closest spiral galaxy to the Milky Way, it is also the farthest object visible to the naked eye from Earth: it is 2.5 million light-years from us.

Together with our own galaxy, it is considered the largest and brightest of the galaxies that make up the Local Group, made up of approximately 30 small galaxies plus three large spiral galaxies: Andromeda, the Milky Way and the Triangulum galaxy. As Andromeda is approaching the position of the Milky Way at about 300 kilometers per second, some specialists maintain that both galaxies will collide in about 5.86 billion years and form an even larger galaxy, in an event called Lactomeda A great galactic dump Now, a team of astronomers led by Renuka Pechetti of Liverpool John Moores University in the United Kingdom and Anil Seth of the University of Utah in the United States has confirmed the discovery of an intermediate-mass black hole within the Andromeda galaxy, which had previously been mistaken for a globular star cluster, known as B023-G078. Actually, scientists verified that B023-G078 is not a globular star cluster but a bare core. According to a press release, specialists call bare or stripped nuclei the remains of tiny galaxies that fall into the networks of other larger galaxies: the outer stars are detached by gravitational force, creating a tiny and dense nucleus that orbits around of the largest galaxy. Precisely, in the center of that nucleus will inhabit a black hole.

Using new observational data provided by the Gemini Observatory and images from the Hubble Space Telescope, astronomers confirmed that B023-G078 is a kind of huge landfill with around 6.2 million solar masses, in which the debris from the galaxies that make up the entire system. In this framework, stars in bare cores are more complex than in globular clusters: these features are seen in B023-G078.

A unique black hole
Because of this, at its center is not a “conventional” black hole, but one of the strangest and most difficult to identify: an intermediate-mass black hole. Usually, astronomers more easily identify black holes of up to 100 solar masses and supermassive black holes that reign in the center of galaxies, with millions of solar masses. In contrast, intermediate-mass black holes have about 100,000 solar masses.

According to the conclusions of the new study, recently published in The Astrophysical Journal, the discovery of the bare nucleus and the intermediate-mass black hole will deepen our understanding of mergers between large galaxies and other smaller ones, a phenomenon that has marked the history of the Universe and will continue to do so in the future.

Detection of a 100,000 M ⊙ black hole in M31’s Most Massive Globular Cluster: A Tidally Stripped Nucleus. Renuka Pechetti et al. The Astrophysical Journal (2022). DOI:https://doi.org/10.3847/1538-4357/ac339f