Intermediate-mass black hole
[2][3][4] 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.
The largest up-to-date sample of intermediate-mass black holes includes 305 candidates[12] selected by sophisticated analysis of one million optical spectra of galaxies collected by the Sloan Digital Sky Survey.
[20] In November 2004 a team of astronomers reported the discovery of GCIRS 13E, the first intermediate-mass black hole in the Milky Way galaxy, orbiting three light-years from Sagittarius A*.
[24] In January 2006 a team led by Philip Kaaret of the University of Iowa announced the discovery of a quasiperiodic oscillation from an intermediate-mass black hole candidate located using NASA's Rossi X-ray Timing Explorer.
[citation needed] In 2009, a team of astronomers led by Sean Farrell discovered HLX-1, an intermediate-mass black hole with a smaller cluster of stars around it,[26] in the galaxy ESO 243–49.
[33] In 2018, the Keio University team found several molecular gas streams orbiting around an invisible object near the galactic center, designated HCN-0.009-0.044, suggested that it is a black hole of 32,000 solar masses and, if so, is the third IMBH discovered in the region.
[34] Observations in 2019 found evidence for a gravitational wave event (GW190521) arising from the merger of two stellar-mass black holes, with masses of 66 and 85 times that of the Sun.
[9][6][7][8] In 2020, astronomers reported the possible finding of an intermediate-mass black hole, named 3XMM J215022.4-055108, in the direction of the Aquarius constellation, about 740 million light years from Earth.
[40] In 2023, an analysis of proper motions of the closest known globular cluster, Messier 4, revealed an excess mass of roughly 800 solar masses in the center, which appears to not be extended, and could thus be considered as kinematic evidence for an IMBH (even if an unusually compact cluster of compact objects, white dwarfs, neutron stars or stellar-mass black holes cannot be completely discounted).
Their environments lack the extreme conditions—i.e., high density and velocities observed at the centers of galaxies—which seemingly lead to the formation of supermassive black holes.
