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Title: Detection of a 100,000 M⊙ black hole in M31’s Most Massive Globular Cluster: A Tidally Stripped Nucleus
Authors: Renuka Pechetti et al.
First Author’s Institution: Liverpool John Moores University, UK
Status: Published in ApJ
Intermediate-Mass Black Holes
Stellar-mass black holes — those with masses of tens of solar masses (M☉) — are thought to result from the collapse of massive stars. The formation of supermassive black holes — those with millions to billions of solar masses — is less clear. Given their large masses, there is not enough time for stellar-mass black holes to grow into the supermassive black holes that we know existed fairly early in the history of the universe. One possibility is that the “seeds” that grow into supermassive black holes lie somewhere in between 102 and 105 M☉, or what we’d call intermediate-mass black holes.
Despite their importance, intermediate-mass black holes remain elusive and their existence has not been quite confirmed. The best way to measure black hole masses is using the motions of stars around them, but this tactic may not work for intermediate-mass black holes since they have a smaller sphere of influence than supermassive black holes. Today’s article takes a look at a possible intermediate-mass black hole in a globular cluster in neighboring galaxy M31, also known as the Andromeda Galaxy.
Pinning Down the Mass
The globular cluster B023-G078 is the most massive cluster in Andromeda, and the velocity of stars within the cluster seems to indicate the presence of a massive central object. The authors of the article use images of the cluster from the Hubble Space Telescope and spectroscopic observations from Gemini to determine if this central mass could be an intermediate-mass black hole.
Figure 1: Root-mean-square velocity of stars in the cluster vs. radial distance to the center of the cluster. Points in red show the observations from the Gemini telescope. The black line shows the best-fit model for a massive black hole. The blue line shows the model assuming there is no black hole. [Adapted from Pechetti et al. 2022]
The best-fit models give the central object a mass of 9 x 104 M☉, placing it firmly in intermediate-mass black hole territory!
It is possible that the central mass is actually several stellar-mass black holes rather than one intermediate-mass black hole. The main difference between the two possibilities would be that a collection of many black holes would look more extended than a single compact object. The authors investigate this possibility using their models, but any conclusions may require higher resolution observations.
However, there is something else that can give us a clue if this is indeed an intermediate-mass black hole: the origin of the globular cluster.
Remnants of a Small Galaxy?
Because of the wide spread of metallicity measured for stars in the cluster, the authors consider the possibility that B023-G078 is the remnant of a small galaxy that underwent a merger with Andromeda, making it a stripped nuclear star cluster. The idea is that as small galaxies merge into larger galaxies (what is known as a minor merger), tidal forces pull apart parts of the galaxy, including the nuclear star cluster at the center that houses a massive black hole, leaving behind a globular cluster.
Given the mass of the cluster (~106 M☉), the authors estimate that the original galaxy had a mass of ~109 M☉. (For comparison, the mass of the Milky Way is ~1011 M☉.) Since the mass of a central black hole typically scales with the mass of the galaxy, this mass estimate means this nucleus is a good place to look for an intermediate-mass black hole.
The combination of the mass of the black hole from modeling and the evidence that this cluster is a stripped nuclear star cluster leads the authors of the article to favor the idea that there is indeed an intermediate-mass black hole in the cluster!
Original astrobite edited by Alex Pizzuto.
About the author, Gloria Fonseca Alvarez:
I’m a fifth-year graduate student at the University of Connecticut. My research focuses on the inner environments of supermassive black holes. I am currently working on measuring black hole properties from the spectral energy distributions of quasars in the Sloan Digital Sky Survey. As a Nicaraguan astronomer, I am also involved in efforts to increase the participation of Central American students in astronomy research.