# Galaxy Shapes Dictated By Central Black Holes? **URL:** \ **Category:** Factual Questions\ **Created:** [July 16, 2013, 2:32pm UTC](https://boards.straightdope.com/t/galaxy-shapes-dictated-by-central-black-holes/663667 "2013-07-16T14:32:49Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![Werekoala](https://avatars.discourse-cdn.com/v4/letter/w/7993a0/32.png) [@Werekoala](https://boards.straightdope.com/u/Werekoala)\ **Post date:** [July 16, 2013, 2:32pm UTC](https://boards.straightdope.com/t/galaxy-shapes-dictated-by-central-black-holes/663667/1 "2013-07-16T14:32:49Z")
I know that it’s fairly accepted that most large galaxies have one or more Supermassive Black Holes at their core. My question is; for spiral galaxies, is their shape dictated or at least influenced by a spinning black hole at the center, or is the galaxy’s spin just part of it’s inherent evolution and/or possibly the cause of the black hole’s formation in the first place (by concentrating more mass at their center)? Followup - if an elliptical or irregular galaxy formed and happened to have a black hole at the center, would it eventually become a spiral if the black hole was spinning? Would it retain it’s elliptical or irregular shape if the black hole were not spinning? ---
**Author:** ![MikeS](https://avatars.discourse-cdn.com/v4/letter/m/919ad9/32.png) [@MikeS](https://boards.straightdope.com/u/MikeS)\ **Post date:** [July 16, 2013, 3:44pm UTC](https://boards.straightdope.com/t/galaxy-shapes-dictated-by-central-black-holes/663667/2 "2013-07-16T15:44:23Z")
I don’t believe that the direction of the spin axis of Sagittarius A\* (the black hole at the center of the Milky Way) is actually known. The most recent article I was able to find was [this one](http://iopscience.iop.org/0004-637X/735/2/110), which basically says that the data are consistent with anywhere between zero spin and spinning quite fast that it’s not a black hole any more. If we can’t measure the Milky Way’s black hole’s spin very well, I doubt that we can do so for other galaxies. So we probably don’t actually know whether the axes of supermassive black holes tend to be aligned with the planes of their host galaxies or not. My impression is that the flattened shape of a spiral galaxy is largely dictated by the angular momentum of the initial concentration of matter that collapsed to form it, and that said angular momentum was determined long before the central black hole formed. But I’m quite rusty on my knowledge of galaxy formation, so take what I say with a grain of salt. ---
**Author:** ![Learjeff](https://avatars.discourse-cdn.com/v4/letter/l/94ad74/32.png) [@Learjeff](https://boards.straightdope.com/u/Learjeff)\ **Post date:** [July 16, 2013, 3:48pm UTC](https://boards.straightdope.com/t/galaxy-shapes-dictated-by-central-black-holes/663667/3 "2013-07-16T15:48:49Z")
> [@Werekoala](#): > > Followup - if an elliptical or irregular galaxy formed and happened to have a black hole at the center, would it eventually become a spiral if the black hole was spinning? Would it retain it’s elliptical or irregular shape if the black hole were not spinning? I can’t answer your first question, but I think the second one has the cart before the horse. Angular momentum is preserved, so the central black hole spins to the extent that the whole galaxy spins (or at least, that portion of it that has collapsed into it so far). I seem to remember reading (probably in wiki articles on galaxy shapes) that big enough galaxies tend to evolve to spirals, and I couldn’t quite follow that. Rather, I can (vaguely) understand how a disk evolves to a spiral, but not the (albeit simpler) idea of how an ellipse would collapse to a disk. The latter only makes sense to me if it gets swallowed up by a black hole and then spewed back out. But that also contradicts the preservation of angular momentum (off the main axis) so clearly I’m missing something very basic here. ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [July 16, 2013, 5:05pm UTC](https://boards.straightdope.com/t/galaxy-shapes-dictated-by-central-black-holes/663667/4 "2013-07-16T17:05:59Z")
> [@MikeS](#): > > If we can’t measure the Milky Way’s black hole’s spin very well, I doubt that we can do so for other galaxies. So we probably don’t actually know whether the axes of supermassive black holes tend to be aligned with the planes of their host galaxies or not. Some galaxies have galactic jets, which gives us some hope. I found a paper, [Jet Directions in Seyfert Galaxies: Radio Continuum Imaging Data](http://iopscience.iop.org/0067-0049/132/2/199/fulltext/52176.text.html), which says in its Introduction that Seyfert Galaxy and black hole axes don’t appear to be aligned: > [@](#): > > Recently Schmitt et al. (1997), Clarke, Kinney, & Pringle (1998), and Nagar & Wilson (1999) showed that there is little or no correlation between the position angle of radio jets and galaxy disk major axes in Seyfert galaxies. Employing a statistical inversion technique, which uses the observed values of (the difference between the position angle of the jet and the host galaxy disk major axis) and i (inclination of the galaxy disk relative to the line of sight), Clarke et al. (1998) and Nagar & Wilson (1999) showed that the observed -distribution can be reproduced by a homogeneous distribution of angles (the angle between the jet and the galaxy disk axis). > > ``` > These results confirm previous ones based on smaller samples (Ulvestad & Wilson 1984b; Brindle et al. 1990; Baum et al. 1993) and contradict the expectation that the jets should be aligned perpendicular to the galaxy disk. Assuming that the accretion disk and black hole are fed by gas from the host galaxy disk, it is natural to expect both disks to be aligned and to have the same angular momentum vector. As a result, since jets are emitted perpendicular to the accretion disk, we would expect them to be aligned with the host galaxy minor axis, which is not observed. These studies give us information about the circumnuclear region of Seyfert galaxies and may be important in the study of processes involved in the feeding of the active galactic nucleus. Possible explanations for the misalignment of the jet and the host galaxy disk are warping of the accretion disk, which can be caused by the self-induced radiation instability (Pringle 1996, 1997; Maloney, Begelman, & Pringle 1996) or by the misaligned inflow of gas from minor mergers (Barnes & Hernquist 1992, 1996). Kinney et al. (2000) give a complete list of possible causes of which we are aware for the observed misalignment. > > ``` They go on to present their measurements, but I couldn’t find an easily digestible table of what they found themselves… ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [July 16, 2013, 5:30pm UTC](https://boards.straightdope.com/t/galaxy-shapes-dictated-by-central-black-holes/663667/5 "2013-07-16T17:30:21Z")
> [@Werekoala](#): > > I know that it’s fairly accepted that most large galaxies have one or more Supermassive Black Holes at their core. My question is; for spiral galaxies, is their shape dictated or at least influenced by a spinning black hole at the center, or is the galaxy’s spin just part of it’s inherent evolution and/or possibly the cause of the black hole’s formation in the first place (by concentrating more mass at their center)? > > Followup - if an elliptical or irregular galaxy formed and happened to have a black hole at the center, would it eventually become a spiral if the black hole was spinning? Would it retain it’s elliptical or irregular shape if the black hole were not spinning? The answer is the black hole’s spin has pretty much no effect, simply because the mechanisms that allow the rotation of a rotating black hole to affect surrounding matter are totally insignificant on a galactic scale. ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [July 16, 2013, 5:40pm UTC](https://boards.straightdope.com/t/galaxy-shapes-dictated-by-central-black-holes/663667/6 "2013-07-16T17:40:53Z")
> [@ZenBeam](#): > > Some galaxies have galactic jets, which gives us some hope. I found a paper, [Jet Directions in Seyfert Galaxies: Radio Continuum Imaging Data](http://iopscience.iop.org/0067-0049/132/2/199/fulltext/52176.text.html), which says in its Introduction that Seyfert Galaxy and black hole axes don’t appear to be aligned: > They go on to present their measurements, but I couldn’t find an easily digestible table of what they found themselves… Note though that it does not necessary follow that the angular momentum of a black hole and the angular momentum of its accretion disk are aligned either. ---
**Author:** ![MikeS](https://avatars.discourse-cdn.com/v4/letter/m/919ad9/32.png) [@MikeS](https://boards.straightdope.com/u/MikeS)\ **Post date:** [July 16, 2013, 6:20pm UTC](https://boards.straightdope.com/t/galaxy-shapes-dictated-by-central-black-holes/663667/7 "2013-07-16T18:20:30Z")
> [@Asympotically\_fat](#): > > Note though that it does not necessary follow that the angular momentum of a black hole and the angular momentum of its accretion disk are aligned either. The inner parts of the accretion disc, at least, are likely to be aligned with the spin axis of the black hole due to the combination of the Lense-Thirring precession and the viscosity of the fluid. (I think this is called the Bardeen-Petterson effect.) But it’s true that this alignment won’t necessarily extend out to the entire accretion disc, so you can end up with a warped disc with one plane near the hole and another out at the rim. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [July 16, 2013, 11:33pm UTC](https://boards.straightdope.com/t/galaxy-shapes-dictated-by-central-black-holes/663667/8 "2013-07-16T23:33:25Z")
> [@](#): > > Quoth **MikeS** : > > I don’t believe that the direction of the spin axis of Sagittarius A\* (the black hole at the center of the Milky Way) is actually known. The most recent article I was able to find was this one, which basically says that the data are consistent with anywhere between zero spin and spinning quite fast that it’s not a black hole any more. It’s generally believed that most astrophysical black holes, of any scale, probably have dimensionless spin parameter quite close to but a little bit less than 1 (the maximum theoretically possible). All of the plausible formation mechanisms favor high spin over low spin. ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [July 17, 2013, 6:45am UTC](https://boards.straightdope.com/t/galaxy-shapes-dictated-by-central-black-holes/663667/9 "2013-07-17T06:45:23Z")
> [@MikeS](#): > > The inner parts of the accretion disc, at least, are likely to be aligned with the spin axis of the black hole due to the combination of the Lense-Thirring precession and the viscosity of the fluid. (I think this is called the Bardeen-Petterson effect.) But it’s true that this alignment won’t necessarily extend out to the entire accretion disc, so you can end up with a warped disc with one plane near the hole and another out at the rim. Interesting, I hand’t heard of the Bardeen-Petterson effect, though I did guess that Lense-Thirring precession may over time cause some sort of alignment between the black hole’s spin and it’s accretion disk. I think I may’ve spoken too soon earlier too, whilst reading up on the Bardeen-Petterson effect I noticed a [paper](http://arxiv.org/abs/0705.4035) that claims that the Bardeen-Petterson effect may be responsible for precession in the jet produced by the accretion of matter by the central supermassive black hole of Messier 106, which may in turn be responsible for the anomalies in its arms. In other words, to answer the OP for a very large galactic centre supermassive black hole, that has a high spin and is active (i.e. accreting a lot of matter) over a long period of time, it is at least plausible that its spin may affect the arms of the host galaxy. ---
**Author:** ![deltasigma](https://avatars.discourse-cdn.com/v4/letter/d/e5b9ba/32.png) [@deltasigma](https://boards.straightdope.com/u/deltasigma)\ **Post date:** [July 17, 2013, 7:48am UTC](https://boards.straightdope.com/t/galaxy-shapes-dictated-by-central-black-holes/663667/10 "2013-07-17T07:48:39Z")
I don’t know how relevant this is, but it’s something new and possibly interesting. - [double disk dark matter](http://physicsworld.com/cws/article/news/2013/jun/03/do-dark-matter-discs-envelop-galaxies) > [@](#): > > In the latest work, Lisa Randall and colleagues at Harvard University argue that such weakly interacting particles might not tell us the whole story. By considering the characteristics of the dark matter surrounding our own Milky Way galaxy, the researchers calculate that as much as 5% of that dark matter might not be weakly interacting. They also point out that this “double-disc dark matter” (DDDM), as they call it, would probably dissipate energy while retaining angular momentum from its motion about the galactic centre, causing it to form a thin disc just as ordinary galactic matter does. They work out that the dark and visible discs would have about the same mass, which would imply that the densities of DDDM and normal matter in the universe would be roughly equal. > “Our model isn’t proposed to solve any particular problems,” says group member Matthew Reece. “But we think it’s important to consider a wider range of possibilities for what dark matter might be. We are lucky to be living in a data-rich era, and we want to be sure that we’re not overlooking a dramatic discovery in all that data.” > > According to the researchers, DDDM discs would contain the dark-matter equivalent of protons and electrons interacting via an analogue of electromagnetism, so creating dark atoms. The minimalistic model that they have considered in the current work, however, does not include analogues of the nuclear forces, so they have not predicted the DDDM spawning stars, as such. Reece says that he and his colleagues limited themselves to this simple model because it was relatively easy to analyse and work out how it could be tested, but explains that a more complex model, incorporating nuclear physics, could in principle be developed. In fact, he adds, “It doesn’t seem completely out of the question that there could be forms of life made of dark matter,” although he underlines that this speculative idea “isn’t very scientific since we don’t know a good way to test it”.