Intermediate-mass black hole
black hole with a mass range of 100 to 100000 solar masses
An intermediate-mass black hole (IMBH) is a class of black hole with mass in the range of 100 to 100,000 (102–105) solar masses: significantly higher than stellar black holes but lower than the hundred thousand to more than one billion (105–109) solar mass supermassive black holes. Several IMBH candidate objects have been discovered in the Milky Way galaxy and others nearby, based on indirect gas cloud velocity and accretion disk spectra observations of various evidentiary strength.
Observational evidence
The gravitational wave signal GW190521, which occurred on 21 May 2019 at 03:02:29 UTC, and was published on 2 September 2020, resulted from the merger of two black holes. They had masses of 85 and 65 solar masses and merged to form a black hole of 142 solar masses, with 8 solar masses radiated away as gravitational waves.
Another gravitational-wave signal (GW231123) from an intermediate mass black hole merger was observed on 23 November 2023 and announced by the LIGO-Virgo-KAGRA collaborations in July 2025.
Before that, the strongest evidence for IMBHs came from a few low-luminosity active galactic nuclei. Due to their activity, these galaxies almost certainly contain accreting black holes, and in some cases the black hole masses can be estimated using the technique of reverberation mapping. For instance, the spiral galaxy NGC 4395 at a distance of about 4 Mpc appears to contain a black hole with mass of about 3.6×105 solar masses.
The largest up-to-date sample of intermediate-mass black holes includes 305 candidates selected by sophisticated analysis of one million optical spectra of galaxies collected by the Sloan Digital Sky Survey. X-ray emission was detected from 10 of these candidates confirming their classification as IMBH.
Some ultraluminous X-ray sources (ULXs) in nearby galaxies are suspected to be IMBHs, with masses of a hundred to a thousand solar masses.
Begin with the source’s own compact description: “Intermediate-mass black hole” is black hole with a mass range of 100 to 100000 solar masses. The dossier treats that line as a proposition to test through Intermediate-mass, black and hole, not as a finished interpretation.
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