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jaibhavaya t1_jdxnpmw wrote

Reply to comment by Nerull in Are galaxies just giant accretion disks around super massive black holes? by darthvadercock

So is it just some coincidence that super massive black holes tend to be at the center of spiral galaxies?

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BailysmmmCreamy t1_jdy089s wrote

Almost certainly not a coincidence, but we don’t know whether the supermassive black holes are the ‘seed’ of the galaxy or whether the galaxy formed first and the stuff in the center collapsed into the black hole.

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Kenshkrix t1_jdyapre wrote

I lean towards the "seed" idea, because black hole formation and growth as we understand it requires that supermassive black holes had to form already extremely massive relative to an 'ordinary' black hole.

A sufficiently energetic collapse to cause a supermassive black hole straight out is extremely unlikely to occur if things are already in any kind of orbit.

We still don't really know, but it's pretty interesting nonetheless.

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jaibhavaya t1_jdz01j3 wrote

I guess that makes sense… if matter wasn’t already coalescing, then what would have collapsed to create the initial black hole? The black hole forms an accretion disk, then the accretion disk pulls other matter into orbit… and suddenly you have a galaxy? So really the “rest” of the galaxy ends up orbiting around its collective center of gravity, that just so happens to be close to the center of this initial big ol’ black hole.

Or something like that? I read that on the underside of a Snapple cap.

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Kenshkrix t1_jdzvo4k wrote

The main issue with black holes is that you can't feed them too much stuff at once unless it falls directly into them, which won't happen in a galactic environment since everything has orbital momentum.

Once a black hole has an accretion disk, the disk itself has so much energy that it will shove away extra matter trying to fall into it.

Thus, one theory on the formation of supermassive black holes is the "black hole star".

Put simply, the idea is that in the early universe in areas where there weren't any particularly big things or galaxies it would be possible for light years worth of diffuse gas to begin accelerating towards the same area, which could collapse directly into a singularity.

It would still have enormous amounts of gas falling towards it, though, and the sheer gravity of all this gas could overcome the energy emitted by the relativistic accretion disk and continue to grow the black hole at a prodigious rate.

Eventually the balance would break and it would explode, but most of the remaining gas might not reach escape velocity, this would be the "seed" of a potential galaxy.

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Makhnos_Tachanka t1_jdygrqt wrote

It's not a coincidence, it's just physics. Any rotating system like this will tend to sort objects by mass, with the heaviest at the middle. You can see this if you just take some sand and swirl it in a cup with some water. The largest grains will end up at the center. Why? They're the heaviest, and it takes the most energy to fling them around. If we simplify a galaxy to a two body system, you will imagine that a light star and a heavy black hole will orbit each other with the star making a much longer orbit than the black hole, which may be almost stationary. In a galaxy, you don't have a two body system, you have every body in the entire galaxy acting together, and relativistic effects and all that, but the same thing is happening. The distribution of masses in a galaxy broadly follows what's called the Einsato profile, which essentially says the densest objects will have the lowest radius. Of course, galaxies come in all shapes and sizes, and come in various states of development (a galaxy that has just collided with another may have a random distribution) but over time, a galaxy will organize itself with the densest objects at its center.

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theboehmer t1_jdxxymc wrote

From what I understand, black holes are incredibly massive. They formed systems that added to the collective mass(gravity), which in turn led to more and more collected mass(stars/dust/what not). Over billions of years it's now the galaxy we know. Our galaxy is in a group of galaxies that will evolve and attract until they merge, growing further.

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jaibhavaya t1_jdy8uyq wrote

Yea! That’s certainly how I understand it. I was curious how that fit into the black hole contributing little to holding the galaxy together as commented. Maybe it just plays a larger part in the early formation of the galaxy.

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ktElwood t1_jdz0o5c wrote

Rather think of them as incredibly dense.

A black hole the mass of earth would be super small, but would have the same "gravitational pull"

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theboehmer t1_jdzluqe wrote

True, it's a little counterintuitive thinking more mass equals smaller radius

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