This one came late to the black hole forum.

I currently favour the model of generation of densly packed globular clusters of metal poor stars undergoing a runaway interstellar traffic jam.

The progenitor stars would initially be at a UV flux driven accretion/condensation maximum mass of ~2.5 solar masses with spectral type around O4e. The stars after formation will continue to fall through the nebula shockfront into the core of the globular cluster along with several hundred thousand others of all masses from 2.5 solar masses down to masses approaching the planet Uranus (Brown dwarves). Meanwhile the nebula shockfront containing mostly H and He will continue to condense several hundred thousand more stars from the gas accreting onto the nebula shockfront shell which will then fall into the core of the globular cluster.

The largest globular clusters will form a runaway accretion supergiant in the core well above the mass of the rest of the stars and will finally become a black hole when the core is over and above the mass of the individual stars landing on the outer layers.

Result is the core evolving to Fe via Gamma cassiopeiae circulation cell formation (infalling equatorial disc and outflowing polar fountain containing the core fusion products.(the results can be seen in the supergiant * Eta Carinae (bipolar hourglass nebula after switching on anotheer nuclear burning stage). The Gamma cassiapieae signature predisposes the star to form a black hole accretion disc and bipolar jets with the rest of the core dumping onto the black hole.

Result is a black hole with somewhere between 20 and 50 solar masses (over & above the 2.5 solar mass max of individual stars in the rest of the cluster and generally 5 to 10* the mass of other collision merger remnant stars in the cluster.

The initial black hole in the core of the cluster then gets dumped on, without being booted out of the cluster by encounters with stars more massive than the black hole.

This mechanism will produce massive black holes in large globular clusters and also galactic nuclei. The only upper limit will be the mass of the gas falling onto the nebula shockfront prior to the formation of the massive supergiant, and also how long it takes for the gas blown away by the newly created black hole & jet system to fall back into the dense cluster.

The result will also produce a rapidly precessing black hole in all 3 axes as a result of the sum of collision and merger of the stars forming the progenitor supergiant star.

The precession axes will only stabilise during a merger of 2 black holes, with a resulting merged remnant larger black hole with very low precession rate typical of large elliptical galaxies and atypical of classical spirals such as the Milky way.

Have a nice day: Ag