Decades of astronomy have revealed that supermassive black holes, weighing up to billions of times the mass of the Sun, inhabit the centers of most galaxies, if not all of them. In some galaxies, these black holes power quasars, in which the energetic matter near the black hole emits copious amounts of light.
This output has helped us spot quasars at great distances, meaning they date from when the Universe was just a few hundred million years old.
That raises an obvious question: how can you build something that big in such a (relatively) short matter of time? A review in today's issue of Science (part of a series of articles dedicated to black holes) describes several potential means for generating a black hole of this size on a very tight schedule.
Most black holes are produced by supernovae, triggered by stars that are less than 100 times the mass of the Sun. These leave behind black holes that are just a few times the solar mass. While these can grow by drawing in material from their environment, it would require a very high rate of growth for a very long time—it's simply not realistic for these to have given rise to the giants at galaxies' cores.
But there are lots of reasons to think they wouldn't need to. Several models of star formation indicate that the first stars were hundreds of times the mass of the Sun, much larger than any stars we're certain exist today. That's because they formed in the absence of heavier elements, which allow for a more efficient star formation by radiating away heat generated by the gravitational collapse of the star. With none of these heavy elements around, stars had to be much heavier to overcome their own heat.
At the high end, these ultra-heavyweights did not have the sort of life cycle we associate with stars in the current Universe. Modern stars find a balance between the inward force of gravity and the outward pressure driven by fusion, and can burn for billions of years. In these early stars, gravity is so large that fusion never has a chance—they simply collapse directly into a black hole that weighs in at around 200 times the mass of the Sun. If enough of these formed in the heart of early galaxies, mergers and a rapid accumulation of gas might be sufficient to grow them rapidly.
A variation on this idea shifts the mergers earlier. In this model, the initial stars don't have to be as large—which may be a good thing, given that some models of star formation are now suggesting lighter stars could have formed in the early Universe. In the dense centers of the earliest galaxies, however, these stars could collide and merge, forming a supermassive star, a few thousand times the mass of the Sun. If this happens rapidly enough, the star could collapse directly into a black hole that is 1,000 times the mass of the Sun.
Finally, the most exotic option being considered is one where dynamic instabilities in a large gas cloud could create a sudden and rapid infall that's fast enough to overcome the outward pressure of fusion. In this case, the core of the star would collapse into a black hole even as more material keeps falling into the body's outer layers. This would create something that's a hollow shell of "star" surrounding a large black hole that would siphon off material just a bit faster than it arrived. By the time the black hole had swallowed the shell entirely, models suggest it could be as massive as a million Suns.
How can we distinguish among these models? One option would be to study more quasars at the most distant edges of the visible Universe, since these would be the oldest and thus the closest to the original formation of their black holes. But the best option would be to have a record of the violent events that produce them. For that, we have to wait for the further development of gravity wave detectors.
None of these mechanisms provide for a black hole with billions of solar masses. But the star forming regions of the early Universe were probably dense enough that collisions among small protogalaxies were common, and these should lead to mergers of these central black holes. But that's something we do have a grip on, as demonstrated by a video of merger simulations that accompanied Science's black hole package.
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