7
$\begingroup$

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?

$\endgroup$
  • 8
    $\begingroup$ Nobody knows for sure because nobody knows for sure how they form. $\endgroup$ – Rob Jeffries Jul 10 '16 at 20:22
  • 1
    $\begingroup$ How supermassive black holes formed in the early Universe is an unsolved problem in physics and astronomy. $\endgroup$ – Sir Cumference Jul 10 '16 at 21:41
  • $\begingroup$ Possible duplicate of Are we moving ever closer to the center of our Galaxy due to a super massive black hole? $\endgroup$ – Fattie Jul 11 '16 at 14:34
  • 2
    $\begingroup$ 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. $\endgroup$ – Fattie Jul 11 '16 at 14:39
  • 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 Jul 11 '16 at 17:09
2
$\begingroup$

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.).

$\endgroup$
0
$\begingroup$

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.

$\endgroup$
  • 2
    $\begingroup$ This addresses some of the gaps in the OP's knowledge, but it does not answer the question. $\endgroup$ – called2voyage Jul 11 '16 at 17:10
  • $\begingroup$ I used a wrong term(edited now), by "pull" I meant the "sphere of influence" in regard to nearby stars. $\endgroup$ – betolink Jul 11 '16 at 19:48
  • 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. $\endgroup$ – Sir Cumference Jul 13 '16 at 13:09

protected by Community Jul 16 '16 at 23:26

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.