Black holes in the centres of galaxies

[davidge]

Why is it believed that almost every big galaxy has a black hole on its center? Does the explanation rely on General Relativity?

 

[kimbyd]

See here:
https://en.wikipedia.org/wiki/Supermassive_black_hole#Outside_the_Milky_Way

Due to the difficulty of detecting most black holes, only a small number have been observed directly. However, the tight relationship between certain properties of the galaxy and the mass of the black holes seems to indicate that the formation of such a supermassive black hole is intimately tied to the formation of the galaxy. This would imply that essentially every galaxy has one.

 

[rootone]

Relativity suggests that black holes can exist, and Einstein himself had a problem with that.
He thought that the math allowed it, but it could not be a real physical thing, there was something wrong with the math.
However the effect of black holes as predicted by GR has actually been observed.
https://en.wikipedia.org/wiki/Gravitational_microlensing
https://arxiv.org/abs/astro-ph/0109467

 

[Vanadium 50]

Why is it believed that almost every big galaxy has a black hole on its center?

Before discussing why something is true we should figure out if it is true. M33 appears not to have a central black hole.

 

[jim mcnamara]

Some interesting pictures and facts about M33:

http://hubblesite.org/explore_astronomy/black_holes/encyc_mod1_q11.html

 

[kimbyd]

Some interesting pictures and facts about M33:

http://hubblesite.org/explore_astronomy/black_holes/encyc_mod1_q11.html

It may just have a small one:
https://arxiv.org/abs/astro-ph/0107359

 

[Vanadium 50]

It may just have a small one:

That doesn't say they see something, or even maybe see something. If M33 has a central black hole, it weighs less than 0.1% of the Milky Way's, and probably less than 0.05%. That's like looking for an elephant and reporting that there's nothing bigger than a pug. Or maybe a chihuahua.

 

[stoomart]

Before discussing why something is true we should figure out of it is true. M33 appears not to have a central black hole.

I understand it's difficult to explain how SMBHs have grown so large in relation to the age of the observable universe, is there any correlation between the size of SMBHs and the expected dark matter in their galaxies?

 

[kimbyd]

That doesn't say they see something, or even maybe see something. If M33 has a central black hole, it weighs less than 0.1% of the Milky Way's, and probably less than 0.05%. That's like looking for an elephant and reporting that there's nothing bigger than a pug. Or maybe a chihuahua.

What they're saying is that the kinematics of the galaxy and a small black hole are consistent with the M-sigma relationship. The M-sigma relationship is the main reason why it's believed nearly all galaxies have supermassive black holes at their centers.

 

[kimbyd]

I understand it's difficult to explain how SMBHs have grown so large in relation to the age of the observable universe, is there any correlation between the size of SMBHs and the expected dark matter in their galaxies?

I don't think there's believed to be any direct relationship between dark matter and the masses of the black holes, for the reason that dark matter doesn't collapse like normal matter does, because it doesn't experience much of any friction. There is likely a loose correlation because more massive galaxies tend to have more dark matter associated with them, and more massive galaxies also tend to have larger black holes at their centers. But it's likely not due to the black holes swallowing dark matter, as the dark matter isn't really able to lose its orbital energy efficiently enough to fall into the black holes (some will, naturally, but not much).

 

[sysprog]

I think that spiriform galaxies. such as ours, look like swirlings around a central drain.

 

[kimbyd]

I think that spiriform galaxies. such as ours, look like swirlings around a central drain.

Perhaps, but that's an illusion. Black holes are only a tiny, tiny part of the masses of galaxies like our own (typically a fraction of a percent). The spiral arms themselves are likely a result of density waves. Here's one popular article that talks about them, and the science that surrounds them:
https://www.scientificamerican.com/article/what-process-creates-and/

The spiral arms are all about how stars interact with one another in the galaxy. The gravity from the black hole at the center doesn't play any significant role (the black hole may play a role if it's active, by blowing away the galaxy's dust that provides the friction which keeps it in a disk shape).

 

[sysprog]

Perhaps, but that's an illusion. Black holes are only a tiny, tiny part of the masses of galaxies like our own (typically a fraction of a percent). The spiral arms themselves are likely a result of density waves. Here's one popular article that talks about them, and the science that surrounds them:
https://www.scientificamerican.com/article/what-process-creates-and/

The spiral arms are all about how stars interact with one another in the galaxy. The gravity from the black hole at the center doesn't play any significant role (the black hole may play a role if it's active, by blowing away the galaxy's dust that provides the friction which keeps it in a disk shape).

That seems to me like saying a singularly identifiable impetus is probably not the only force affecting or effecting a result -- here's an obvious analogy: an open drain in a half-full bathtub doesn't entirely account for all the observable fluid dynamics of the swirling phenomena occurring therein -- it's just a part of an explanation of why the water doesn't present as pristinely still.

 

[kimbyd]

That seems to me like saying a singularly identifiable impetus is probably not the only force affecting or effecting a result -- here's an obvious analogy: an open drain in a half-full bathtub doesn't entirely account for all the observable fluid dynamics of the swirling phenomena occurring therein -- it's just a part of an explanation of why the water doesn't present as pristinely still.

No.

The spiral arms form due to the gravitational dynamics of the stars within the arms.

The black hole's gravity has basically nothing to do with it, because the black hole's gravity is minuscule compared to the galaxy as a whole.

Where the black hole has an effect, it comes from the fact that the black hole at the center of the galaxy can get extremely bright when it's swallowing matter at a rapid pace. It can get so bright that it can blow nearly all of the gas and dust out of a galaxy, which removes the friction which keeps the galaxy in a disk shape, resulting in the galaxy becoming spheroidal and having no significant structure at all.

 

[sysprog]

No.

What specifically in my post are you saying "no" to ?

The spiral arms form due to the gravitational dynamics of the stars within the arms.

That seems plausible to me -- even so, I still think that the central anchor point being a super-massive black hole is a determinative factor in the overall phenomenon set.

The black hole's gravity has basically nothing to do with it, because the black hole's gravity is minuscule compared to the galaxy as a whole.

The mass of the central SMBH, despite being small compared to that of the entire galaxy, has extreme density compared to other intra-galactic aggregates of mass, and is vastly more massive than other mass aggregates (stars) in its vicinity. That, I suppose, is signifintly factorial regarding why smaller-mass objects swirl around it, rather than it around them, just as the moon orbits the earth, and the earth-moon system orbits the sun. I understand that the mass of the sun is much greater than that of all the planets combined, and that the mass of the SMBH near the mass-center of the galaxy is much less than that of the entire galaxy; however, the centrallly-located SMBH appears to be the tipping-point around which the balance of the galaxy revolves -- not especially unlike water swirling around and toward a drain.

Where the black hole has an effect, it comes from the fact that the black hole at the center of the galaxy ...

Please, given that you apparently disagree with me regarding the matter, explain your theory regarding why the "center of the galaxy" is occupied by a super-massive black hole, instead of by empty space, or by a less massive star..

... can get extremely bright when it's swallowing matter at a rapid pace. It can get so bright that it can blow nearly all of the gas and dust out of a galaxy,

Are you sure that most of the gas and dust so impacted escapes the galaxy, and does not in passing accrete unto intra-galactic objects, such as stars?

... which removes the friction which keeps the galaxy in a disk shape, resulting in the galaxy becoming spheroidal and having no significant structure at all.

Isn't alignment of angular momenta involved? Don't we already employ our understandings of fluid dynamics in ideational modelling of formation and evolution of galaxies? I still think that spiriform galaxies that have a central SMBH (as far as I know, all spiriform galaxies that have been observed systematically have appeared to have such an object at or near center of mass) look like swirlings around a drain.

 

[Vanadium 50]

If you are arguing that it looks like "swirlings around a drain" like Ursa Major looks like a big bear, I think nobody is arguing with you. If you are arguing that the trajectories of stars follow trajectories like "swirlings around a drain", that is not what is observed.

 

[sysprog]

If you are arguing that it looks like "swirlings around a drain" like Ursa Major looks like a big bear, I think nobody is arguing with you. If you are arguing that the trajectories of stars follow trajectories like "swirlings around a drain", that is not what is observed.

My view is somewhere in between those -- I think that the macro-scale rotatory centrality and apparent angular momenta of the masses within spiriform galaxies that have a SMBH at or near center of mass is analogous to water swirling about a drain, and that eventually all the matter in the galaxy that does not attain galactic escape velocity will at some point become within the accretion range of the SMBH, and so become part thereof, just as, analogously, all the water in a vessel will, in time, as inexorably as gravity directs, flow down an open drain therein -- in the case of the galaxy we inhabit, it is predicted that the nearest other galaxy (Andromeda), also spiriform, also with a central SMBH, will likely catastrophically collide with ours long before our or its central SMBH accretes anything anywhere near what is now in our or its outermore region ...

 

[Bandersnatch]

My view is somewhere in between those -- I think that the macro-scale rotatory centrality and apparent angular momenta of the masses within spiriform galaxies that have a SMBH at or near center of mass is analogous to water swirling about a drain, and that eventually all the matter in the galaxy that does not attain galactic escape velocity will at some point become within the accretion range of the SMBH, and so become part thereof, just as, analogously, all the water in a vessel will, in time, as inexorably as gravity directs, flow down an open drain therein -- in the case of the galaxy we inhabit, it is predicted that the nearest other galaxy (Andromeda), also spiriform, also with a central SMBH, will likely catastrophically collide with ours long before our or its central SMBH accretes anything anywhere near what is now in our or its outermore region ...

Galaxies are not giant accretion discs for their central black holes. They are not feeding their stars to their BHs.
The stars are on stable orbits, with no global galactocentric velocity component. Once the material in the immediate proximity of the black hole is accreted, i.e. once a galaxy passes its active nucleus phase early in its life, there is no reason for any star to fall into the black hole, apart from being very unfortunately slingshot into what is a rather tiny target.
A black hole of 1 million Sun's mass from 1000 ly has as much gravitational influence on orbiting stars as the Sun has on its environment from 1 ly away. Which is pretty meh, and way too low to keep the stars at that distance in orbit considering their velocities. At 4000 ly from the black hole, a star has as much reason to orbit it or fall into it as the Alpha Centauri system has to do the same w/r to the Sun.
Which is to say, that stars behave in their orbits like they do due to the collective gravitational influence of other stars and gas, not of the black hole.
You'd get density waves forming spiral patterns regardless of whether there is or isn't a black hole in the centre, because that's not what is causing them. They just look like they're spiralling down, but the individual orbits are stable.

 

[sysprog]

Galaxies are not giant accretion discs for their central black holes. They are not feeding their stars to their BHs.
The stars are on stable orbits, with no global galactocentric velocity component. Once the material in the immediate proximity of the black hole is accreted, i.e. once a galaxy passes its active nucleus phase early in its life, there is no reason for any star to fall into the black hole, apart from being very unfortunately slingshot into what is a rather tiny target.
A black hole of 1 million Sun's mass from 1000 ly has as much gravitational influence on orbiting stars as the Sun has on its environment from 1 ly away. Which is pretty meh, and way too low to keep the stars at that distance in orbit considering their velocities. At 4000 ly from the black hole, a star has as much reason to orbit it or fall into it as the Alpha Centauri system has to do the same w/r to the Sun.
Which is to say, that stars behave in their orbits like they do due to the collective gravitational influence of other stars and gas, not of the black hole.
You'd get density waves forming spiral patterns regardless of whether there is or isn't a black hole in the centre, because that's not what is causing them. They just look like they're spiralling down, but the individual orbits are stable.

A set of stably orbiting objects around a galactic center is not a perpetual motion system. The fact that their, albeit variously elliptical, and inter-influenced, orbits are, in sum, approximately centered on or near the SMBH at or near the galactic center of mass, indicates that when the orbits eventually decay, they will, in individual and group order, do so in the direction of the SMBH around which they are orbiting.

 

[Vanadium 50]

I'm sorry, but Bandersnatch is right. Galaxies are not giant accretion discs for their central black holes. They are not feeding their stars to their BHs. Some spirals, like M33, appear not even to have a supermassive black hole in their center.

 

[kimbyd]

What specifically in my post are you saying "no" to ?

That the gravity of supermassive black holes has a large effect on the dynamics of stars in the galaxy. It doesn't.

You seem to have the common misconception that a black hole sucks in matter. This is false. If you replaced our Sun with a black hole of the same mass, the Earth's orbit would not be changed at all.

The only effect of the black hole's gravity comes from its mass, and that mass is minuscule compared to the stars in the galaxy once you move beyond the immediate galactic core. The spiral arms are found much further out. If the spiral arms reach very close to the center of the galaxy, that galaxy likely has a very small black hole (this is the essence of the M-sigma relation that has been discussed in the thread).

The mass of the central SMBH, despite being small compared to that of the entire galaxy, has extreme density compared to other intra-galactic aggregates of mass, and is vastly more massive than other mass aggregates (stars) in its vicinity.

Only in a very small region around the black hole. The spiral arms do not extend that close to the center. In fact, there's an inverse relationship between how close the spiral arms get to the center and the mass of the black hole: galaxies with larger black holes have larger "bulges", and the spiral arms only occur outside of the bulge. A galaxy's bulge is the spheroidal-shaped group of stars that make up the galaxy's center.

Are you sure that most of the gas and dust so impacted escapes the galaxy, and does not in passing accrete unto intra-galactic objects, such as stars?

Yes, I'm sure. Accretion requires a loss of energy so that the gas and dust can collapse, while the process of removing the dust adds energy. Though the process of removing most of the dust from the galaxy may result in some star formation (due to the collisions of dust clouds), in the main the heating of the dust clouds will tend to reduce the number of stars that form.

Isn't alignment of angular momenta involved? Don't we already employ our understandings of fluid dynamics in ideational modelling of formation and evolution of galaxies? I still think that spiriform galaxies that have a central SMBH (as far as I know, all spiriform galaxies that have been observed systematically have appeared to have such an object at or near center of mass) look like swirlings around a drain.

The more massive the black hole, the less "spiral-shaped" the galaxy becomes. The really massive black holes tend to be at the centers of elliptical galaxies.

 

[sysprog]

Thanks for your elucidations, kimbyd, Vanadium 50, and Bandersnatch -- I think that the central SMBH in a spiriform galaxy owes the centrality of its location to its super-massivity -- even though it may be of extremely low mass in comparison to the whole of the galaxy -- I acknowledge the interesting point that a galaxy can be spiriform without any SMBH or other super-massive object at its center, and further, I acknowledge that greater central masses will entend galactic formations more toward spheroidicity than toward discoidosity; however, predicated upon basic understanding of a three-body system of unequally massive objects, I think that spiriform galaxies tend to establish centralities of orbits around their most massive high-density objects.

 

[kimbyd]

Thanks for your elucidations, kimbyd, Vanadium 50, and Bandersnatch -- I think that the central SMBH in a spiriform galaxy owes the centrality of its location to its super-massivity -- even though it may be minuscule in comparison to the whole of the galaxy -- I acknowledge the interesting point that a galaxy can be spiriform without any SMBH or other super-massive object at its center, and further, I acknowledge that greater central masses will entend the galactic formation more toward spheroidicity than toward discoidosity; however, predicated upon basic understanding of a three-body system of unequally massive objects, I think that spiriform galaxies tend to establish centralities of orbits around their most massive high-density objects.

Three things.
1. Centrality of orbit is a different issue entirely from whether or not the galaxy is a spiral galaxy. The stars in elliptical galaxies also tend to orbit the centers of those galaxies. They just have orbits that are in all directions, rather than mostly confined to a disk. Spiral galaxies have orbits that are confined to a disk because of the friction of gas and dust, and the spiral arms form due to the way those stars interact with one another.
2. The supermassive black hole doesn't determine the center of mass of the galaxy. A supermassive black hole may settle near the center of mass, but it will tend to be a small fraction of the galaxy's mass, so that most of the stars won't really be orbiting the supermassive black hole at all: they'll be orbiting a different point that is determined by the center of mass of all the stars and gas and dust and dark matter that is contained within their orbits. The black hole itself may be close to that center, but won't determine the center's location, since for stars that are more than a short distance from the center all that other stuff will be more massive.
3. Unlike our solar system, with nearly all of its mass concentrated within the Sun, the galaxy has its mass distributed throughout. This means that the orbits of individual stars don't tend to be the simple elliptical orbits that we see. They can be very irregular, as the stars get attracted by local stars and nebulae that they pass near along their complicated orbits.

 

[fumbar]

Before discussing why something is true we should figure out of it is true. M33 appears not to have a central black hole.

And that is why people say "almost every." They seem to be present in a large fraction of major galaxies.

 

[fumbar]

That doesn't say they see something, or even maybe see something. If M33 has a central black hole, it weighs less than 0.1% of the Milky Way's, and probably less than 0.05%. That's like looking for an elephant and reporting that there's nothing bigger than a pug. Or maybe a chihuahua.

This isn't all that uncommon in observational astronomy. Its really tough to get an idea of what is going on.

 

[fumbar]

I still think that the central anchor point being a super-massive black hole is a determinative factor in the overall phenomenon set.

This is not how orbits work in galaxies. Every star is attracted to every other star in the galaxy. (Plus the dark matter halo, which we will ignore for now.) A SMBH can put 10,000,000 solar masses into the volume of our solar system, which is a much larger average density than the rest of galaxy. However, if pick a star at random from a galaxy of 1 trillion stars and then draw a line from my star to the central black hole and then center an arc of width 3.6 arcseconds (1/1000th the way around a full circle) on that line the arc will enclose at the very least 1/1000th of the stars in the galaxy, which for a galaxy of 1 trillion stars is 1 billion stars. The 10 million solar mass black hole then constitutes (1/100th) of the mass in the smallest possible slice which is itself only 1 of 1,000. The SMBH is such a small portion of the total mass that you could likely remove it and see minimal change in the large scale structure of the galaxy.Assumptions in my argument: Spiral galaxies can be treated as 2 dimensional disks. The average mass of a star is 1 solar mass. A spiral galaxy cut into "slices" like a pizza would have equal amounts of stars per slice.

 

[Vanadium 50]

And that is why people say "almost every." They seem to be present in a large fraction of major galaxies.

I disagree with that. Or rather, I believe it may be true, but that the evidence is not strong enough to declare this a fact.
What is known?

1. Out of the few dozen SMBH's discovered in spirals, they all occur in galaxies with central bulges.
2. There is an empirical relationship between the size of the central bulge and, if it has been detected, the size of the SMBH. This is purely empirical: there is no generl agreement that this is caused by known galaxy dynamics, although there are conjectures.

I don't believe from 1 and 2 that the conclusion "All galaxies with central bulges have SMBH's" follows. It may be true, but it does not logically follow. I'd like to see one of two lines of evidence - either a dynamic explanation of M-sigma that shows SMBH's are inevitable, or a statement that given a known and calibrated SMBH-finding efficiency, the number of observed SMBH's is consistent with 100% of the bulgy spirals having them.

Then there is the other fact
3. Not all spirals have central bulges. M33, for example. The number without is maybe 20%? 25%? To me, that would mean most spirals have bulges, but not almost every spiral has a bulge.

 

[jedishrfu]

Here's an article from the Universe Today online magazine with Fraser Crain answering the question:

https://www.universetoday.com/13732/why-are-there-black-holes-in-the-middle-of-galaxies/

Short answer is Astronomers just don't know why but have a couple of scenarios to consider:
- the black hole was there and the galaxy formed around it
- a portion of the galaxy collapsed to form the black hole