Observational Evidence
While previously there had been no certain evidence of the existence of intermediate-mass black holes, a team at the CSIRO radio telescope in Australia announced on 9 July 2012 that it had discovered the first intermediate-mass black hole.
Some ultra-luminous X ray sources (ULXs) in nearby galaxies are suspected to be IMBHs, with masses of a hundred to a thousand solar masses. The ULXs are observed in star-forming regions (e.g., in starburst galaxy M82), and are seemingly associated with young star clusters which are also observed in these regions. However, only a dynamical mass measurement from the analysis of the optical spectrum of the companion star can unveil the presence of an IMBH as the compact accretor of the ULX.
Additional evidence for the existence of IMBHs can be obtained from observation of gravitational radiation, emitted by the compact remnant that orbits the IMBH. Also, the M-sigma relation predicts the existence of black holes with masses of 104 to 106 solar masses in low-luminosity galaxies.
They are too massive to be formed by the collapse of a single star, which is how stellar black holes are thought to form. 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. There are three popular formation scenarios for IMBHs. The first, is the merging of stellar mass black holes and other compact objects by means of accretion. The second one is the runaway collision of massive stars in dense stellar clusters and the collapse of the collision product into an IMBH. The third is that they are primordial black holes formed in the big bang.
Read more about this topic: Intermediate-mass Black Hole
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