A black hole with a mass of 800 million solar masses would have a event horizon radius of about 16 astronomical units. That would be about the distance between the Sun and somewhere between the orbits of Saturn and Uranus. Now that is a big black hole, but is not like a vacuum cleaner that would “suck up” the entire galaxy it is in. In fact, it would only “suck up” the matter which happens to be very near the black hole.
800 million solar masses is not a large number compared to a whole galaxy. For example, our Milky Way galaxy contains 300 billion stars, and the largest galaxies can have around 6 trillion stars. The gravitational effect that this black hole has on its galaxy as a whole is no greater than if you replaced this black hole with 800 million indivdual stars. Stars and gas clouds that would be in a stable orbit around 800 million indivdual stars near the center of the galaxy would also be in a stable orbit around this black hole.
That black hole was accreting mass then, 13 billion years ago, because that is how we were able to detect it now and to estimate its mass. These early universe massive black holes are detected as quasars that are producing a huge amount of luminosity due to the process of the accretion of mass - if I remember correctly, something like 40% of the rest mass of the infalling mass is converted to energy by processes in the accretion disk around the black hole.
However, once the quasar clears out all the mass in its immediate environment, it would go quiescent and essentially become invisible. In fact we know that all quasars eventually go quiescent since the peak number of quasars are found in the early universe. Wikipedia reports (Wikipedia) that “quasar discovery surveys have demonstrated that quasar activity was more common in the distant past. The peak epoch of quasar activity in the Universe corresponds to redshifts around 2, or approximately 10 billion years ago.” If quasars did not go quiescent, the number of visible quasars would always be increasing with time.
Throughout time, an occasional star may get perturbed to fall into the vicinity of the black hole and there would be another brief period of high luminosity, but these events would be infrequent. Another, more significant way for a quiescent supermassive black holes to become reactivated as a quasar is when galaxies merge. Gas clouds that were orbiting the two supermassive black holes in the two galaxies will collide and can then fall into the black holes producing a quasar. This could reactivate the quasar for a much longer period of time since most of a galaxy’s ordinary matter is in the form of gas clouds rather than stars. In addition, when galaxies merge the individual stars rarely collide, whereas gas clouds often collide (which we can see as bursts of star formation in merging galaxies). These gas clouds collisions can also disrupt the gas cloud orbits and can cause them to “fall” closer to the black holes where they can then form accretion disks around the supermassive black holes producing reactivated quasars.
Again, I am not an expert, but since quasars eventually go quiescent, I predict that the mass today would not be very significantly above the mass it had 13 billion years ago.
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