intermediate-mass black hole

The term intermediate-mass black hole (IMBH) was coined for any black hole that is more massive than a stellar-mass black hole (i.e., a black hole formed from collapse of a star), but not as massive as a supermassive black hole (SMBH). One cited criteria is a mass in the range of 100 to 100,000 solar masses. Theories of how such black holes might have formed:

• through black hole mergers of stellar-mass black holes. The possibility of subsequent mergers has been explored as well.
• formed through some early-universe process, i.e., primordial black holes in that mass range.
• formed through stellar mergers within the dense region of a globular cluster, e.g., during a globular cluster's core collapse.

The LIGO/Virgo GW detections revealed black holes larger than anticipated, i.e., large for stellar-mass black holes, some of them contradicting some theoretical limits on their mass (into what was considered the upper mass gap of stellar-mass black holes). One detection, GW190521 produced a black hole of 142 solar masses, which qualifies as an IMBH given the above criteria.