Producing Black Holes via Runaway Mergers

In the cores of young dense star clusters (YDSCs), stars are tightly packed in very small regions of space. Dynamical interactions between stars can cause them to collide and merge with one another. The first pair of stars to collide and merge form what is referred to as the principal collision product (PCP). The more massive a PCP gets through subsequent mergers, the more efficient it becomes in participating in further mergers. This creates a runaway collision scenario that can lead to the formation of a very massive star. The very massive star can then collapse to form an intermediate massive black hole (IMBH). An IMBH is basically a black hole whose mass is larger than stellar mass black holes but smaller than supermassive black holes.

The metallicity of a YDSC can determine whether or not an IMBH can form. Metallicity is simply the abundance of elements heavier than hydrogen and helium. Mapelli (2016) show that for a YDSC with the same metallicity as the Sun, the very massive star generated from the runaway collision scenario loses so much mass through powerful stellar winds that the resulting black hole that is formed is no more than ~30 times the mass of the Sun. At low metallicity (i.e. 0.01 to 0.1 times the Sun's metallicity), the rate of mass loss becomes much lower and the resulting black hole can be up to a few hundred times the mass of the Sun, well within the mass range of IMBHs.

Reference:
Mapelli (2016), "Massive black hole binaries from runaway collisions: the impact of metallicity", arXiv:1604.03559 [astro-ph.GA]