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It is worth considering that a black hole's hold on its galaxy is not as strong as a sun-like star's on its accretion disk. Our Sun is 99% of the mass of the so

It is worth considering that a black hole's hold on its galaxy is not as strong as a sun-like star's on its accretion disk. Our Sun is 99% of the mass of the solar system, our Sagittarius A* is ~4 million solar masses, while our Milky Way is 800 billion solar masses. On top of that, black hole growth rate slows to nearly a stop at ~270 billion solar masses, so even if a black hole manages to meet the rare conditions to get that heavy, it would still be dwarfed by its galaxy.

Moldoteck 3y ago
Is the upper limit due to hawking radiation?
- adgjlsfhk1 3y ago
  it isn't. hawking radiation slows as the black hole size increases. I believe it's more just that it's very unlikely for too much mass to pass by a black hole
  - Moldoteck 3y ago
    wow, i've assumed that if bh increases, ther's more space for howking radiation to escape so bh will loose more mass... I guess physics and intuition aren't going that well together in some cases)
- dotnet00 3y ago
  Something to do with the spin rate and the stability of the accretion disk. I don't fully understand it, just something I picked up off wikipedia.
brabel 3y ago
But at the same time, I think it's been shown the the size of a galaxy almost always reflects the size of its central black hole... that is, the influence of the central black hole seems to be way, way bigger than what you would expect from the black hole's gravity alone.
- antonvs 3y ago
  That doesn't follow. A galaxy is not like a star system that orbits a single massive object in a single plane. Objects orbit the center of mass of the galaxy, which is made of of an enormous mass of stars and clouds of dust and gas. For example, the central molecular zone (CMZ) is an asymmetrical roughly spherical region, about 1600-1900 light years in diameter, that contains about 60 million solar masses of gas and dust alone, not counting the stars in that region. That's 15 times heavier than the central black hole, or put another way, the black hole is less than 6% of its mass. By contrast, the Sun is more than 99% of the mass in the solar system - the planets are rounding errors by comparison. With the central black hole, it's the other way around. And that CMZ gas is only about 5% of the gas in the galaxy. There's over 26,000 light years between us and the black hole, and every star and gas cloud in that enormous volume exerts a gravitational influence on us. Zooming out a bit further, the galactic bulge is on the order of 15 billion solar masses, with a roughly spherical radius of about 6500 light years. The central black hole is less than 0.03% of that mass. If you want to imagine us and other stars in the spiral arms as orbiting a single central object, that bulge would be a better choice. But as I said, there's still another 23,000 light years between us and that bulge, filled with stars whose gravitational influence we feel. If anything, causation is likely to work the other way around, in that larger galaxies have larger central black holes because they have denser central regions. The questions that exist are more around how the central black hole affects galaxy formation, a sort of which came first question.
  - somat 3y ago
    An excellent reply, far better than my idle musings warranted, however a point was missed. At some point in solar system formation the mass of the not quite yet a sun was only 6% of the mass of the entire system. At some very early point I assume the distribution of mass in the proto solar dust cloud would be the same as the distribution of mass in the galaxy. I hesitate to reinforce my idea because really, it is ill informed musings on how solar system formation is coalescing gas and dust clouds and would galaxy sized objects eventually coalesce into the same proportion of objects. ill-informed because it turns out we can look into the past and see what newer galaxies look like. Anyway, thank you again for a great post.

Moldoteck 3y ago

Is the upper limit due to hawking radiation?

adgjlsfhk1 3y ago
it isn't. hawking radiation slows as the black hole size increases. I believe it's more just that it's very unlikely for too much mass to pass by a black hole
- Moldoteck 3y ago
  wow, i've assumed that if bh increases, ther's more space for howking radiation to escape so bh will loose more mass... I guess physics and intuition aren't going that well together in some cases)
dotnet00 3y ago
Something to do with the spin rate and the stability of the accretion disk. I don't fully understand it, just something I picked up off wikipedia.

brabel 3y ago

But at the same time, I think it's been shown the the size of a galaxy almost always reflects the size of its central black hole... that is, the influence of the central black hole seems to be way, way bigger than what you would expect from the black hole's gravity alone.

antonvs 3y ago
That doesn't follow. A galaxy is not like a star system that orbits a single massive object in a single plane. Objects orbit the center of mass of the galaxy, which is made of of an enormous mass of stars and clouds of dust and gas. For example, the central molecular zone (CMZ) is an asymmetrical roughly spherical region, about 1600-1900 light years in diameter, that contains about 60 million solar masses of gas and dust alone, not counting the stars in that region. That's 15 times heavier than the central black hole, or put another way, the black hole is less than 6% of its mass. By contrast, the Sun is more than 99% of the mass in the solar system - the planets are rounding errors by comparison. With the central black hole, it's the other way around. And that CMZ gas is only about 5% of the gas in the galaxy. There's over 26,000 light years between us and the black hole, and every star and gas cloud in that enormous volume exerts a gravitational influence on us. Zooming out a bit further, the galactic bulge is on the order of 15 billion solar masses, with a roughly spherical radius of about 6500 light years. The central black hole is less than 0.03% of that mass. If you want to imagine us and other stars in the spiral arms as orbiting a single central object, that bulge would be a better choice. But as I said, there's still another 23,000 light years between us and that bulge, filled with stars whose gravitational influence we feel. If anything, causation is likely to work the other way around, in that larger galaxies have larger central black holes because they have denser central regions. The questions that exist are more around how the central black hole affects galaxy formation, a sort of which came first question.
- somat 3y ago
  An excellent reply, far better than my idle musings warranted, however a point was missed. At some point in solar system formation the mass of the not quite yet a sun was only 6% of the mass of the entire system. At some very early point I assume the distribution of mass in the proto solar dust cloud would be the same as the distribution of mass in the galaxy. I hesitate to reinforce my idea because really, it is ill informed musings on how solar system formation is coalescing gas and dust clouds and would galaxy sized objects eventually coalesce into the same proportion of objects. ill-informed because it turns out we can look into the past and see what newer galaxies look like. Anyway, thank you again for a great post.