Strange ‘Missing Link’ Black Hole Found in the Andromeda Galaxy

Black holes – like any other holes – should come in all sizes. Most galaxies have a supermassive black hole at their center. At the other end of the black hole size chart are those formed when small stars collapse. In the middle of those two extremes, researchers believe there should be intermediate black holes but they haven’t been able to find one to explain how or why they should exist … until now.

“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."

In a press release announcing a new study published in The Astrophysical Journal, lead author Renuka Pechetti of Liverpool John Moores University describes the discovery of the first of its kind intermediate-mass black hole (IMBH), which was discovered hiding inside an enormous star cluster B023-G07 in the Andromeda galaxy. Andromeda is the Milky Way’s closet neighbor and strange discoveries are nothing new for it – in 2018, astronomers determined that Milky Way once had a twin galaxy that was consumed by Andromeda. Could B023-G07 be a remnant of that heinous act? Anil Seth, associate professor of astronomy at the University of Utah and co-author of the study, thinks it might be the product of a similar act of galactic cannibalism.

“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. 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."

B023-G078 was previously thought to be a globular cluster – a tight spherical collection of stars whose origins are poorly understood by astronomers but have predicable characteristics … like a signature light profile that has the same shape near the center or at the outer edge. New observational data from the Gemini Observatory and images from the Hubble Space Telescope show B023-G078 is different enough that the research team decided it is a stripped nucleus – a remnant of a small galaxies that fell into Andromeda and had its outer stars stripped away, leaving a tiny dense cluster surrounding … something. Simulations showed that the ‘something’ was a strange black hole that was most likely the remnant of the swallowed smaller galaxy’s supermassive black hole. That would make it an intermediate-mass black hole – in this case, a 100,000-solar-mass black hole -- and the act of Andromeda cannibalizing a galaxy is the reason for its existence.

In astronomy, once you know how to find one of an object, you have the key to finding more – and that’s what astronomers will be doing. A good next candidate is the large globular cluster Omega Centauri in our own Milky Way galaxy, which astronomers speculate may surround a smaller IMBH of 1,000-solar-masses. There are also three other globular clusters in Andromeda with potential, and then there are all the new ones to be discovered as soon as the James Webb Space Telescope becomes operational.

Is this enough to forgive Andromeda for being a galactic cannibal?