How are Intermediate mass black holes formed?

Black holes are divided, based on their mass, into supermassive black holes (SMBHs) and stellar black holes. The mechanism that creates the later, is well understood however, the origin of SMBHs remains uncertain. One possible scenario that has been proposed is that supermassive black holes are formed from smaller intermediate mass black holes (IMBHs) that merge together. But then, how are IMBHs formed? A new study tries to shed light to this question.