According to https://en.wikipedia.org/wiki/Sphere_of_influence_(black_hole) the gravitational sphere of influence of a supermassive black hole is really limited compared to the size of its hosting galaxy, why then is there one at the center of every galaxy?
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10$\begingroup$ Nobody knows for sure because nobody knows for sure how they form. $\endgroup$– ProfRobCommented Jul 10, 2016 at 20:22
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2$\begingroup$ How supermassive black holes formed in the early Universe is an unsolved problem in physics and astronomy. $\endgroup$– Sir CumferenceCommented Jul 10, 2016 at 21:41
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3$\begingroup$ betolink - that wikipedia article is so confusing I would delete it. They are not talking about "influence" as you are thinking; it just means influence in comparison to certain other technical matters. This recent answer is terrific. astronomy.stackexchange.com/a/16233/13071 Note that a BH of mass X, affects things around it exactly the same as any other thing of mass X; black holes are not special in any way. They don't "suck things in". If our sun was replaced by a BH of the identical mass: absolutely nothing, at all, would happen to Earth. Earth would orbit identically. $\endgroup$– FattieCommented Jul 11, 2016 at 14:39
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2$\begingroup$ @JoeBlow This is not a duplicate of the linked question. A misunderstanding may have lead the user to ask the question and the question may be an unsolved problem, but it is nevertheless a valid question. $\endgroup$– called2voyage ♦Commented Jul 11, 2016 at 17:09
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1$\begingroup$ @JoeBlow and everyone else, it is indeed chicken and egg. There is every possibility that central black holes do have a major influence over the galaxies they exist in through the winds and ionising radation caused by quasar activity. In that sense they are not equivalent to other things of the same mass. SMBHs are uniquely capable of powering quasars and converting rest mass into other forms of energy. $\endgroup$– ProfRobCommented Jul 16, 2016 at 23:23
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2 Answers
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As some of the comments pointed out, there isn't a definite answer since this is still ongoing research. However, there are some things that might explain it:
Galaxy formation: Current theory says that dark matter is non-uniformly distributed in the early universe. Once radiation and baryonic matter decouple, matter flows towards the dark matter overdensities. We know that stars (Pop III) must form very early during this, because their light reionizes the universe. These early stars are thought to have a mass of up to 1000 solar masses. It's possible that some of the infalling gas collapses straight into a black hole.
Density profile: A galaxies' density rises sharply towards the centre. So, during the formation of the galaxy, the gas rushing inward might have formed a black hole without going through the process of star and supernova. Also, this means there's enough to feed the black hole once it's formed.
Dynamic friction: Imagine a self-gravitating system of heavy and light objects (like a star cluster or, indeed, a galaxy). A heavy object passing through an ensemble of lighter ones will lose some of it's kinetic energy to them, which means it will "sink" towards the centre of the gravitational well. This is well observed in globular clusters. Problem: The timescale for this to happen is far too long to matter for large galaxies.
There seems to be a correlation between the mass of the central black hole and the mass of the elliptical galaxy or the bulge of the spiral galaxy. So, some process makes sure that the central black hole gets to grow along with its galaxy.
Coming back to your question: There are two parts to it, one: why a black hole at all, two, why at the centre. The second part is simpler, it would get there eventually, and probably (let's say in a galaxy merger) quite quickly. For the first part, there is no definite answer but the points above are some indications as to why it's not too surprising that there should be one.
As a last point, black holes at the centres of other galaxies are not at all easy to find. Unless the black hole is active (quasar), there are at best very indirect detection methods (velocity dispersion in the central part etc.).
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The gravitational "pull" of a black hole is not limited. In fact the gravitational force of attraction of every particle in the Universe effects every other particle in the Universe, irrespective of their distance of separation or mass. Or to be more exact every particle distorts the fabric of time/space throughout the Universe. The distortion may me small the further away from the particle you get but it is still there according to General Relativity.
Quantum theory will put a of limit on things, in that the when the distortion is reduced to the Plank length it will be lost in the general background noise.
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2$\begingroup$ This addresses some of the gaps in the OP's knowledge, but it does not answer the question. $\endgroup$– called2voyage ♦Commented Jul 11, 2016 at 17:10
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$\begingroup$ I used a wrong term(edited now), by "pull" I meant the "sphere of influence" in regard to nearby stars. $\endgroup$– betolinkCommented Jul 11, 2016 at 19:48
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2$\begingroup$ "The gravitational force of attraction of every particle in the Universe effects every other particle in the Universe, irrespective of their distance of separation or mass." That is not necessarily true. Gravitational waves only travel at the speed of light, so if an object lies outside of our observable universe, its gravity likely hasn't affected us yet. $\endgroup$ Commented Jul 13, 2016 at 13:09