Supermassive black holes and the neutron star connection
Black holes are formed when large stars reach the end of their life. The heat generated within an active (energy-producing) star keeps the ball of gas in balance with the crushing force of gravity, but once all sources of energy are depleted there is nothing to prevent further collapse of the star. A star somewhat bigger than the size of the sun will collapse to a size where gravity is balanced by the electron degeneracy pressure - a white dwarf. With somewhat larger stars, the gravity is too great for this and the star will continue to collapse to a size determined by the neutron degeneracy pressure to form a neutron star. Larger still and there is no known alternative to the star collapsing to form a black hole. This is thought to be when the initial mass is of the order of 10 times the mass of our Sun although the precise figure is not known yet.
Stars come in a range of masses from less than one solar mass to (rarely) a hundred solar masses. During a collapse, a portion of the mass will be lost in a supernova explosion. The eventual range of masses will then be from 0.5 to 30 solar masses. Angular momentum is conserved during a collapse, so just like a skater in a spin pulling their arms into their body, the speed of rotation will increase substantially for newly created black holes. This should result in stellar mass black holes coming into existence already close to being at their maximal spin and thus close to their maximum size.
Our galaxy is host to millions of black holes in this range but also hosts one supermassive black hole at the centre with a mass of a million suns. Other galaxies, and probably all galaxies, also host a supermassive black hole at their centre; some with masses of a billion or more suns. The source of these behemoths remains a puzzle.
It is suggested here that black holes might never coalesce. Even if they occasionally could, the extremal limit would be reached long before these massive sizes were reached.
So the question of just how supermassive black holes can ever form remains an open question. Current theories talk about the possibility of extremely large stars collapsing at end of life in a very early Universe, but numerical simulation has cast this into serious doubt. From that point, they still need to grow at a stupendous rate. And yet supermassive black holes are found to exist in the first 800 million years of the universe's existence.
So where do supermassive black holes really come from?
Although it is argued here that black holes are thought to never coalesce, this is not true for neutron stars. Two counter-rotating neutron stars could combine to form a new black hole, but occasionally with negligible net angular velocity. This resultant black hole could then grow slowly to ultimately form a supermassive black hole with a massive extremal (mass) limit, possibly explaining the existence of supermassive black holes and the relative rarity of intermediate size black holes.
All measurements to date show supermassive black holes with the spin over halfway to the extremal limit, but admittedly for a relatively small sample so far.