There are many “stellar-mass” black holes in the universe, up to tens of times the mass of the sun (M☉), which are formed by the collapse of massive stars. There are also easier-to-observe “supermassive” black holes, millions to billions of times the sun’s size, located at the centers of galaxies, the cause of which has not yet been determined. Due to the huge mass difference between the two, basically stellar-mass black holes do not have enough time to evolve into supermassive black holes that existed in the early universe. There is an argument that the “seed” for the formation of supermassive black holes is between 102~5 M☉, which is the so-called “intermediate mass” black hole.
Although potentially an important key to the evolution of supermassive black holes, intermediate-mass black holes have yet to be proven. The best way to measure the mass of a black hole is to observe the motion of surrounding stars, but it is not applicable to intermediate-mass black holes, because the gravitational influence range is smaller than that of super-massive black holes, and it does not have companion star behavior for reference like stellar-mass black holes. The authenticity of intermediate-mass black holes is the direction that scientists have been striving for in recent years.
Recently, a research team used the Hubble Space Telescope and the Gemini Observatory Spectrometer to study the most massive star cluster B023-G078 in the Andromeda Galaxy (M31), and to analyze whether the dense and massive objects in the center of the cluster are intermediate-mass black holes by stellar motion. The team used Jeans anisotropic modeling to calculate the velocities of several stars within a cluster, depending on the mass and size of the central object. The best fitting model found has a mass of 9×104 M☉, which is the range of intermediate-mass black holes.
But the research team cannot rule out that the central mass is produced by a multi-stellar-mass black hole, which can be verified using a higher-resolution spectrometer. The stars in the B023-G078 cluster have a wide distribution of metallicity, and the team believes that it may be the remnant of a small galaxy that merged with Andromeda to form a stripped nuclear star cluster. Combining the observations of stellar abundance with the central mass of the stellar velocity fit, the research team tends to believe that the cluster does indeed have an intermediate-mass black hole.