### Do all galaxies have a black hole in the center?

• Would I be correct if I stated that all galaxies have a black hole in the center? Since other galaxies orbit around a center point in a galaxy - the barycenter, right? - I would assume at this point there must be some insane gravitational pull. Which only black holes can create, right?

(If this is a bit vague, please tell me so I can clarify!)

I am a complete newbie to astronomy in general, so sorry if this question makes no sense whatsoever.

Why do you say that it "must have some insane gravitational pull"? Stuff orbits the barycenter because it's the center of mass, it doesn't have to be a big concentration of mass. FWIW, for galaxies with a central BH, the mass of that BH is a tiny fraction of the whole galaxy's mass. It's not like the BH dominates the galaxy's gravitational structure, although of course it has a fairly big effect in the immediate vicinity of the BH.

Where's the black hole in the middle of every binary system that makes the stars orbit the barycenter? The mass of the stars is more than enough, you don't need them to orbit something else. Solve for a binary star system, and you'll see the same mechanism still works for n-bodies.

@PM2Ring: While that's a fairly well-known physical result, it ignores the fundamental question: Would all that mass even _be_ there concentrated in a galaxy without a black hole? Or are black holes essential to the formation of starts and their clustering into a galaxy?

@Luaan If the mass of the stars is more than enough, does that mean that they will collide/merge?

@MSalters Sure, central black holes might be important in galaxy formation, but I don't see how they are relevant to star formation. And then you have to explain spiral galaxies without a central BH. Did they lose their BH, eg in a collision? If so, how did they manage to retain their spiral structure? FWIW, we still don't have a good theory to explain how super massive BHs are so massive. I suspect that we need to learn more about the role of dark matter to address these questions properly.

@AndrewMorton If you only account for gravity and ignore gravitational waves, no - the orbits are stable. The real system is trickier, because stars aren't inert, perfectly solid balls of stuff - they're losing large amounts of mass to solar winds, for example. I'm not sure, but I think that main sequence binary stars are actually slowly _expanding_ their orbits as the orbiting mass decreases. It gets much more complicated once you add things like solar evolution, of course, but the "spiraling down to the center" thing needs some way of losing massive amounts of orbital energy.

Does an approximation of the shell theorem apply to the distribution of mass in a galaxy?

4 years ago

The answer here is certainly no. Many dwarf galaxies (e.g. the Magellanic Clouds) don't contain supermassive black holes, although some may contain less massive intermediate-mass black holes (Mezcua et al. 2018). As they are the most numerous type of galaxy in the universe, it's quite possible that most galaxies, in fact, do not contain supermassive black holes. This shouldn't be too surprising; if the bottom-up theories of supermassive black hole formation are correct, many intermediate-mass black holes must coalesce to form a massive one. Dwarf galaxies simply wouldn't have (or be able to form) the requisite population, in most cases.

That said, a number of dwarf galaxies have been observed to host supermassive black holes (notably including M60-UCD1 and Hen 2-10, among others). These have provided, and continue to provide, insights into the formation and evolution of low-mass galaxies.

Some users here have also identified some massive galaxies without supermassive black holes, including M33 and A2261-BCG. The former has a bulge, but the central stellar velocity dispersion is low, implying there is no black hole at its core (Merritt et al. 2001), assuming the required parameters are known well enough. In A2261-BCG, it's possible that one or more central supermassive black holes were ejected, leaving a "puffy" core without a black hole. Keep in mind, of course, that this is only one possible explanation for the observed distribution of stars in A2261-BCG.

In general, a central supermassive black hole does not influence the large-scale kinematics of stars and gas within a galaxy. Most supermassive black holes are much less massive than their host galaxy; I believe Sag A*, at the center of the Milky Way, is on the order of about 0.001% of the total mass of the galaxy. Even massive ellipticals like NGC 4889 have black holes about 0.1% of their total mass. As far as I know, ultra-compact dwarfs have the highest black-hole-to-galaxy mass ratios, with an extreme example being the previously-mentioned M60-UCD1.

the idea of ejected SMBHs is an intriguing one, given that this would imply such objects wandering about in between galaxies will little chance of being detected.

@Michael It's definitely an interesting prospect. That said, ejected active galactic nuclei would still be detectable, as they're usually much more luminous than their host galaxies, and because they should be able to retain accretion disks for some time post-ejection. HE0450-2958 *might* be such an ejected AGN (although the naked quasar interpretation is not well-accepted), and SDSS J0927+2943 and 3C 186 are AGN that have likely been ejected, but are still detectable.

It's worth noting the original post didn't ask about *supermassive* black holes. Just regular old black holes. Do we mean to say that there are *no* black holes at the center of Magellanic clouds?