Re: How would the massive black holes that exist in galaxies have been produced

Excellent question - by which I mean that it’s one that astronomers can’t answer yet! In the past several years, astronomers have discovered that most galaxies (to be precise, those with stellar bulges) have massive black holes in the center. Furthermore, the mass of these black holes increases with the mass of the stellar bulge. For example, the black hole in the center of the Milky Way, which has a relatively small bulge, is two million times as massive as our sun. The black hole in M87, one of the largest nearby galaxies, is four billion solar masses!

So where did these black holes come from, and how did they get so large? We do know that about half of the mass was accumulated through a process called “accretion.” The centers of galaxies tend to be pretty violent places, with lots of gas streaming around. Some of this gas ends up close to the black hole and is pulled into it, which makes the black hole grow. At the same time, some of the matter’s energy is released through electromagnetic radiation – that is, light. Accretion actually releases quite a lot of energy, and we see these systems as “quasars” or other types of “active galactic nuclei.” By comparing the amount of quasar light we see to the number of black holes, astronomers have found that about half of the black hole mass grew through accretion.

It’s possible that nearly all of the quasar mass was accumulated in this way. In such a scenario, black holes could form as gas falls into the centers of the first galaxies. However, recent numerical simulations have shown that this is actually difficult, because (except in extremely unusual circumstances) the gas would form stars as it falls in.

As a result, we probably need some sort of “seed” black hole. One way to get these is from stars. As you know, massive stars can form black holes when they die. We’ve observed several such black holes in our Galaxy; all of them have masses about five to ten times that of the sun. It’s unlikely that normal stars can make black holes much larger than this. As stars get more massive, they become hotter and produce more and more light. For stars of composition similar to the sun, this light essentially gets trapped in the star and pushes the envelope out. Once the star reaches 100 times the mass of the sun, this outward force becomes overwhelming. Even for stars less massive than this limit, most of the stellar material is lost before the black hole forms, leaving a remnant of order ten solar masses. So the black holes from normal stars are probably not big enough to be seeds for the supermassive black holes we see in the centers of galaxies.

However, there is one way around this. It turns out that the first generation of stars were very different from our sun, because they had no heavy elements. (Heavy elements are formed inside stars, so by definition this generation can’t begin with them!) In this case the stars can grow to be much more massive, because the light is able to escape more easily and exerts a smaller outward push on the star’s envelope. In principle, these stars could be hundreds or even a thousand solar masses, and their relic black holes could have comparable sizes. These would be large enough to serve as seeds for later accretion, and it is an elegant solution. However, there’s one problem: nobody has ever seen such a star! This is because they must have formed long ago, and such stars would live for only a brief time. We are hoping that future telescopes, like the James Webb Space Telescope will find such objects and tell us whether they really were this massive.

The other way to form a seed is to have a very dense star cluster. As the stars interact with each other, some fraction of them will collapse to the center and become a black hole. Again, we don’t know if this actually happens in the early universe. The star clusters would have to be much denser than anything nearby, but it is possible. New telescopes will certainly be searching for these objects as well.

As I mentioned, this is a very active area of research right now. Hopefully in a few years we will have found more answers!