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The fact is, many, if not most large galaxies have a supermassive black hole at their center. My question is why? Is it because when these galaxies were first formed supermassive black holes were created out of super-dense clouds of matter, which then attracted the rest of the galaxy? Or is the black hole attracted to the center of the galaxy in such a way that due to its large mass and ability to perturb everything around it, the interactions with the galaxy tend to move the center to where the black hole is located?

It seems interesting that Sagittarius A* is at the precise center of the galaxy, when it has so much less mass than the rest of the galaxy put together. The sun being at the center of the solar system makes a lot of sense - it accounts for 99.8% of the mass of the solar system. But the mass of Sagittarius A* is on the order of one millionth the mass of the galaxy.

So, to phrase it like a chicken and egg problem – what came first, the super-massive black holes, or the galaxies surrounding them?

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This is a very well posed question, thanks! The problem is, we still don't know. What we suppose, is that dark matter came first.

Primordial fluctuations of dark matter made the right conditions to accrete enough mass to build SMBHs, and then the surrounding galaxy.

But still we don't know if, the SMBH comes from merging, of other SMBHs or galaxies, or from high massive BH that are called seed (of the order of $100\,M_{Sun}$). Perhaps, there are very recent developments, which could be known by the experts, but until few years ago, this was the scenario.

About the mass issue, we could have a better reasoning if we think to the influence spheres. We discovered a relation between the SMBH mass and many bulge properties (i.e., the luminosity, the velocity dispersion, and others). This means that an object of the order of $\sim1$ pc, influences an object of the order of $\sim1$ kpc (maybe $\sim3$). Plus, the bulge structure is essential to the formation and characterization and classification of the host galaxy.

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  • $\begingroup$ Interesting that the mass correlates so strongly to galactic bulge. Certainly that points to the idea that the formation of SMBHs and galaxies are inter-related somehow, even if it doesn't answer the question one way or the other regarding which came first. $\endgroup$
    – Foo Barrigno
    Commented Jul 1, 2014 at 11:20
  • $\begingroup$ If supermassive black holes had formed from galaxies and not black hole mergers then surely there would be a certain minimum number of stars still orbiting the supermassive black hole and a computer simulation could give an estimate. $\endgroup$
    – user50918
    Commented Jun 1, 2023 at 18:39

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