<p>It's generally assumed that <em>most</em> galaxies have a black hole at their centre. There are galaxies we've detected that don't seem to have a black hole though, so it's not a necessary requirement or a guaranteed find in every case.</p>

<p>In cases without a black hole, it has been postulated that the gravity of the stars and gases combined is enough to hold the galaxy together.</p>

CC BY-SA 4.0
2018-10-15T07:08:39.600Z
Even in galaxies withy central black holes, the gravity of the stars and gas (and dark matter) is enough to hold it together; the black hole is almost always a very small fraction of the total. (E.g., the Milky Way's central black hole is about one ten-thousandth of the combined mass of the Milky Way's stars.)
Peter Erwin
CC BY-SA 4.0
2018-10-15T07:42:46.940Z
Some references would be nice.
pela
CC BY-SA 4.0
2018-10-15T08:41:56.973Z
[Abell 2261](http://iopscience.iop.org/article/10.3847/1538-4357/aa9064/meta)
Dhruv Saxena
CC BY-SA 4.0
2018-10-15T10:56:02.693Z
Upper limit of 1500 solar masses on any possible BH at center of M33: http://iopscience.iop.org/article/10.1086/323481/meta
Peter Erwin
CC BY-SA 4.0
2018-10-15T11:35:36.897Z
You'd think a black-hole-less galaxy would be roughly as rare as a starless planetary system (of our solar-system's mass). Yeah, there's probably some out there, but gravity + the occasional collision + enough time, and enough of the system is bound to makes its way to the center of mass.
T.E.D.
CC BY-SA 4.0
2018-10-15T12:20:53.460Z
@T.E.D. The theory will likely have to be revisited, yes. But that could take anywhere between a day and a millennia. So, for now, we work with the data we got :-)
Mast
CC BY-SA 4.0
2018-10-15T13:26:25.590Z
@T.E.D. That analogy doesn't quite work, because we _know_ that the black holes in the centers of galaxies are just a tiny part of the mass of the whole galaxy. On the other hand, planetary systems are probably utterly dominated by the mass of the star(s) - and our solar system is already far more massive than most star systems. Of course, our observations of other planetary systems are still very limited, so there might be some surprises. But we already know you can have complex "planetary systems" without a star - just look at Jupiter or Saturn.
Luaan
CC BY-SA 4.0
2018-10-15T18:24:10.873Z
I think most galaxies - which are dwarf galaxies, *don't* have black holes.
ProfRob
CC BY-SA 4.0
2018-10-15T23:02:06.227Z
Relevant xckd: https://what-if.xkcd.com/83/, relevant to both what @Luaan and Rob Jeffries said, our solar system is more massive than most star systems only because most star systems are red dwarfs.
Michael
CC BY-SA 4.0
2018-10-16T04:30:03.073Z
A further problem with this answer (and the question itself) is the failure to distinguish between *stellar* black holes - which will be found anywhere that their progenitor stars exist(ed) - and *supermassive black holes* (SMBH) that are generally involved in active galactic nuclei (AGN). In this list of the [galaxies in our local neighbourhood](https://en.wikipedia.org/wiki/List_of_nearest_galaxies), only a small fraction have SMBH, whereas I'm guessing nearly all of them have stellar BH.
Chappo Hasn't Forgotten
CC BY-SA 4.0
2018-10-16T20:03:08.843Z
The answer is misleading. It implies that supermassive black holes are an important part of what holds galaxies together. They are not. The contribution from the central black hole is very tiny.
Florin Andrei
CC BY-SA 4.0
2018-10-14T21:03:05.863Z
2018-10-14T21:03:05.863Z
Kyle
<p>The answer here is certainly no. Many dwarf galaxies (e.g. the <a href="https://en.wikipedia.org/wiki/Magellanic_Clouds" rel="noreferrer">Magellanic Clouds</a>) don't contain supermassive black holes, although <a href="https://www.nasa.gov/mission_pages/chandra/images/cosmos-legacy-survey-shows-evidence-of-intermediate-mass-black-holes.html" rel="noreferrer">some may contain less massive intermediate-mass black holes</a> (<a href="https://arxiv.org/abs/1802.01567" rel="noreferrer">Mezcua et al. 2018</a>). As they are the most numerous type of galaxy in the universe, it's quite possible that <em>most</em> 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.</p>

<p>That said, a number of dwarf galaxies have been observed to host supermassive black holes (notably including <a href="https://en.wikipedia.org/wiki/M60-UCD1" rel="noreferrer">M60-UCD1</a> and <a href="https://en.wikipedia.org/wiki/Hen_2-10" rel="noreferrer">Hen 2-10</a>, among others). These have provided, and continue to provide, insights into the formation and evolution of low-mass galaxies.</p>

<p>Some users here have also identified some massive galaxies without supermassive black holes, including <a href="https://en.wikipedia.org/wiki/Triangulum_Galaxy" rel="noreferrer">M33</a> and <a href="https://en.wikipedia.org/wiki/A2261-BCG" rel="noreferrer">A2261-BCG</a>. The former has a bulge, but the central stellar velocity dispersion is low, implying there is no black hole at its core (<a href="https://arxiv.org/abs/astro-ph/0107359" rel="noreferrer">Merritt et al. 2001</a>), assuming the required parameters are known well enough.  In A2261-BCG, it's possible that <a href="https://www.nasa.gov/mission_pages/hubble/science/a2261-bcg.html" rel="noreferrer">one or more central supermassive black holes were ejected</a>, 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.</p>

<p>In general, a central supermassive black hole does <em>not</em> 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 <a href="https://en.wikipedia.org/wiki/NGC_4889" rel="noreferrer">NGC 4889</a> 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.</p>

CC BY-SA 4.0
2018-10-15T22:58:40.607Z
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
CC BY-SA 4.0
2018-10-15T23:21:23.177Z
@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](https://en.wikipedia.org/wiki/HE0450-2958) *might* be such an ejected AGN (although the naked quasar interpretation is not well-accepted), and [SDSS J0927+2943](https://en.wikipedia.org/wiki/SDSS_J0927%2B2943) and 3C 186 are AGN that have likely been ejected, but are still detectable.
HDE 226868
CC BY-SA 4.0
2018-10-16T21:59:14.547Z
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?
S. Imp
CC BY-SA 4.0
2018-10-15T14:22:36.730Z
2018-10-15T18:47:24.337Z
2018-10-15T18:47:24.337Z
HDE 226868
<p>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? </p>

<p>(If this is a bit vague, please tell me so I can clarify!)<br>
I am a complete newbie to astronomy in general, so sorry if this question makes no sense whatsoever.</p>

CC BY-SA 4.0
2018-10-14T23:36:45.627Z
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.
PM 2Ring
CC BY-SA 4.0
2018-10-15T07:22:07.927Z
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.
Luaan
CC BY-SA 4.0
2018-10-15T11:44:21.110Z
@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?
MSalters
CC BY-SA 4.0
2018-10-15T11:59:41.780Z
@Luaan If the mass of the stars is more than enough, does that mean that they will collide/merge?
Andrew Morton
CC BY-SA 4.0
2018-10-15T12:37:27.600Z
@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.
PM 2Ring
CC BY-SA 4.0
2018-10-15T13:22:13.747Z
@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.
Luaan
CC BY-SA 4.0
2018-10-15T22:50:06.803Z
Does an approximation of the shell theorem apply to the distribution of mass in a galaxy?
Michael
<black-hole><galaxy><galaxy-center>
Do all galaxies have a black hole in the center?
FutureCake
# Do all galaxies have a black hole in the center?

(Asked by: username_0 on 2018-10-14)

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.

**Tags:** black-hole, galaxy, galaxy-center

> **Comments:**
> **username_1**: 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.  
> **username_2**: 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.  
> **username_3**: @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?  
> **username_4**: @username_2 If the mass of the stars is more than enough, does that mean that they will collide/merge?  
> **username_1**: @username_3 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.  
> **username_2**: @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.  
> **username_5**: Does an approximation of the shell theorem apply to the distribution of mass in a galaxy?  

---

## 2 answers

### Answer by username_15 on 2018-10-15. Score: 12 | Accepted answer

The answer here is certainly no. Many dwarf galaxies (e.g. the [Magellanic Clouds](https://en.wikipedia.org/wiki/Magellanic_Clouds)) don't contain supermassive black holes, although [some may contain less massive intermediate-mass black holes](https://www.nasa.gov/mission_pages/chandra/images/cosmos-legacy-survey-shows-evidence-of-intermediate-mass-black-holes.html) ([Mezcua et al. 2018](https://arxiv.org/abs/1802.01567)). 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](https://en.wikipedia.org/wiki/M60-UCD1) and [Hen 2-10](https://en.wikipedia.org/wiki/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](https://en.wikipedia.org/wiki/Triangulum_Galaxy) and [A2261-BCG](https://en.wikipedia.org/wiki/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](https://arxiv.org/abs/astro-ph/0107359)), assuming the required parameters are known well enough. In A2261-BCG, it's possible that [one or more central supermassive black holes were ejected](https://www.nasa.gov/mission_pages/hubble/science/a2261-bcg.html), 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](https://en.wikipedia.org/wiki/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.

> **Comments:**
> **username_5**: 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.  
> **username_15**: @username_5 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](https://en.wikipedia.org/wiki/HE0450-2958) *might* be such an ejected AGN (although the naked quasar interpretation is not well-accepted), and [SDSS J0927+2943](https://en.wikipedia.org/wiki/SDSS_J0927%2B2943) and 3C 186 are AGN that have likely been ejected, but are still detectable.  
> **username_16**: 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?  

---

### Answer by username_6 on 2018-10-14. Score: 6

It's generally assumed that *most* galaxies have a black hole at their centre. There are galaxies we've detected that don't seem to have a black hole though, so it's not a necessary requirement or a guaranteed find in every case.

In cases without a black hole, it has been postulated that the gravity of the stars and gases combined is enough to hold the galaxy together.

> **Comments:**
> **username_7**: Even in galaxies withy central black holes, the gravity of the stars and gas (and dark matter) is enough to hold it together; the black hole is almost always a very small fraction of the total. (E.g., the Milky Way's central black hole is about one ten-thousandth of the combined mass of the Milky Way's stars.)  
> **username_8**: Some references would be nice.  
> **username_9**: [Abell 2261](http://iopscience.iop.org/article/10.3847/1538-4357/aa9064/meta)  
> **username_7**: Upper limit of 1500 solar masses on any possible BH at center of M33: http://iopscience.iop.org/article/10.1086/323481/meta  
> **username_10**: You'd think a black-hole-less galaxy would be roughly as rare as a starless planetary system (of our solar-system's mass). Yeah, there's probably some out there, but gravity + the occasional collision + enough time, and enough of the system is bound to makes its way to the center of mass.  
> **username_11**: @username_10 The theory will likely have to be revisited, yes. But that could take anywhere between a day and a millennia. So, for now, we work with the data we got :-)  
> **username_2**: @username_10 That analogy doesn't quite work, because we _know_ that the black holes in the centers of galaxies are just a tiny part of the mass of the whole galaxy. On the other hand, planetary systems are probably utterly dominated by the mass of the star(s) - and our solar system is already far more massive than most star systems. Of course, our observations of other planetary systems are still very limited, so there might be some surprises. But we already know you can have complex "planetary systems" without a star - just look at Jupiter or Saturn.  
> **username_12**: I think most galaxies - which are dwarf galaxies, *don't* have black holes.  
> **username_5**: Relevant xckd: https://what-if.xkcd.com/83/, relevant to both what @username_2 and Rob Jeffries said, our solar system is more massive than most star systems only because most star systems are red dwarfs.  
> **username_13**: A further problem with this answer (and the question itself) is the failure to distinguish between *stellar* black holes - which will be found anywhere that their progenitor stars exist(ed) - and *supermassive black holes* (SMBH) that are generally involved in active galactic nuclei (AGN). In this list of the [galaxies in our local neighbourhood](https://en.wikipedia.org/wiki/List_of_nearest_galaxies), only a small fraction have SMBH, whereas I'm guessing nearly all of them have stellar BH.  
> **username_14**: The answer is misleading. It implies that supermassive black holes are an important part of what holds galaxies together. They are not. The contribution from the central black hole is very tiny.  

---

