Spiral Rotation in Disk Galaxies: Exploring the Physical Basis of Inference

[TrickyDicky]

If disk galaxies redshifts don't fit normal rotational curves, why do we still think they are spinning? What is the physical basis of that inference? Just because they have the form of a disk?

 

[kungfuscious]

Good question! Yes, when we look at the spectrum of most galaxies, it's true that the lines appear to be red shifted. This led to the whole concept that if almost everything is going away from us, then that is pretty good evidence for the universe expanding.

However, if you look more carefully at a spectrum of a galaxy, especially if it's seen partly edge-on, you can tell about rotation. This is done by taking a spectrum of the light emitted from let's say the LEFT side of the galaxy. You'd still see the overall redshift caused by the entire galaxy moving away from you, but you could for example see that it is slightly MORE redshifted. That would mean that on the left side, it's going away from you a bit more. Then if you take a spectrum of the right side of the galaxy, you see that it's redshifted less. That means you can infer that it's coming towards you a bit more. From this we can tell about the rotation speed of a galaxy. You can do this only for light being emitted, though, which means you miss out on any of the stuff that's NOT giving off light.

That leads to evidence for dark matter existing, because the rotation properties of the visible stuff we can see (stars and gas) show that they don't have nearly enough mass to keep the galaxy rotating the way it is. There must be something else there with mass that's keeping it together - thus leading to dark matter.

 

[TrickyDicky]

Good question! Yes, when we look at the spectrum of most galaxies, it's true that the lines appear to be red shifted. This led to the whole concept that if almost everything is going away from us, then that is pretty good evidence for the universe expanding.

However, if you look more carefully at a spectrum of a galaxy, especially if it's seen partly edge-on, you can tell about rotation. This is done by taking a spectrum of the light emitted from let's say the LEFT side of the galaxy. You'd still see the overall redshift caused by the entire galaxy moving away from you, but you could for example see that it is slightly MORE redshifted. That would mean that on the left side, it's going away from you a bit more. Then if you take a spectrum of the right side of the galaxy, you see that it's redshifted less. That means you can infer that it's coming towards you a bit more. From this we can tell about the rotation speed of a galaxy. You can do this only for light being emitted, though, which means you miss out on any of the stuff that's NOT giving off light.

That leads to evidence for dark matter existing, because the rotation properties of the visible stuff we can see (stars and gas) show that they don't have nearly enough mass to keep the galaxy rotating the way it is. There must be something else there with mass that's keeping it together - thus leading to dark matter.

What you explain about the differences in redshift between the sides of the galaxy does not necesarily mean the galaxy is spinning, it means there is different movement in different parts of the spiral arms along the line of sight from the galaxy to us, the real fact is that the redshift measures don't indicate rotation as it is evident from the flat velocity curves. So is it not more sensible to realize that the galaxy might not be spinning than to make up some new type of matter?

 

[kungfuscious]

The differing redshifts on the left and right side of a galaxy do actually tell you that it is rotating. You can see that it is symmetric on either side. The galaxies are most definitely rotating.

The dark matter is inferred by the flatness of the rotation curves. The speed of a star or gas when going around the galaxy is given by the gravitational field of the matter enclosed within the orbit. The matter outside the orbit actually has no effect (it all cancels out). The speeds of the outermost things we can see (on the rotation curve, the speed of the farthest distances) therefore give us an indication of the matter enclosed within it. The interesting thing is that rotation curves are actually flat. This implies that the further out you go, the MORE matter there is at the edges. The curves sometimes aren't just flat - they increase with distance. This is puzzling, because at the edges of what we can detect (at the outer edge of the galaxy), it's mainly hot gas. It has very few stars. And when we have the problem that when you figure out the mass that the galaxy must have as calculated from the rotation curve, you see that it is much greater than any estimates for the amount of gas and stars that are there.

The galaxy would rotate much differently if the mass was just due to stars in keplerian orbits (they'd be faster in the center, and slower on the outside).

This is very strong evidence for the existence of matter that is there (we can see its effects on the stuff we CAN see, like stars and gas), but that we can't actually see. The name Dark Matter is a bit cheap, because it can be made up of many different things. It could be MACHOs, which could include brown dwarves, black holes, and also things like comets and asteroids. Machos are made of baryonic matter (normal stuff). But even estimates as to the number of them fall far short of what must be there. That's where the term WIMP comes in (weakly interacting massive particles). These are more exotic particles that should have mass but don't interact much. Dark matter researchers consider them as essentially collisionless. Many particles are candidates for WIMPs, from the more well known and now detected Neutrinos, to more exotic and weird theoretical particles like Axions and SUSY (supersymmetric) particles. Those haven't been detected yet. Basically, they're just trying to figure out what the dark matter particles are.

 

[TrickyDicky]

The differing redshifts on the left and right side of a galaxy do actually tell you that it is rotating. You can see that it is symmetric on either side. The galaxies are most definitely rotating.

Not necesarily, other types of spiral arms trajectories could produce that symmetry (which is actually not a perfect symmetry). You might also want to explain how come there are galaxies with arms "rotating" in opposite directions. http://heritage.stsci.edu/2002/03/caption.html

The dark matter is inferred by the flatness of the rotation curves.

Exactly, It's an inference for lack of a less absurd explanation. Anyway, I was trying to engage somebody in a thought experiment, not to recite something that is in every cosmology textbook.

 

[kungfuscious]

I was just trying to answer your questions as clearly as I could. No need to get testy. I have always been puzzled by opposite rotations, though. For example, I took images of NGC7479, a face on spiral galaxy. In optical and NIR wavelengths, it rotates clearly in one direction. However, in radio it rotates in the opposite direction. That's really odd, and I have no idea what physical mechanism could cause it. Rotation of matter in opposite directions should end up causing massive amounts of hot gas. Our images of H-alpha only showed large emission areas that would be expected due to the visible arms. So where is the extra hot gas caused by the collisions of matter moving in opposite directions?

Here's the image we took:
http://lh6.ggpht.com/_wAoQHYUGBNU/THxQUG_95PI/AAAAAAAAHI8/nnKtfMhlhPI/s576/NGC7479 BVHa.jpg

 

[TrickyDicky]

I have always been puzzled by opposite rotations, though. For example, I took images of NGC7479, a face on spiral galaxy. In optical and NIR wavelengths, it rotates clearly in one direction. However, in radio it rotates in the opposite direction. That's really odd, and I have no idea what physical mechanism could cause it. Rotation of matter in opposite directions should end up causing massive amounts of hot gas. Our images of H-alpha only showed large emission areas that would be expected due to the visible arms. So where is the extra hot gas caused by the collisions of matter moving in opposite directions?

Here's the image we took:
http://lh6.ggpht.com/_wAoQHYUGBNU/THxQUG_95PI/AAAAAAAAHI8/nnKtfMhlhPI/s576/NGC7479 BVHa.jpg

Hey, that's the same question I've been asking since I first saw pictures of apparently opposite rotations, so far no one seems to ave come up with a reasonable explanation.
I'm so glad that someone else is also puzzled by this observations, the usual reaction is to ignore them because they don't fit in the mainstream paradigm. BTW, that's a beautiful picture you took.
All I'm saying is that one needs to consider alternative scenarios to try and solve some otherwise extrange observations, even if just to discard them altogether, but the first step is to check them out.

 

[kungfuscious]

The particular galaxy I took a picture of has asymmetry in its spiral structure. Maybe it's due to a recent merger? I have read that some scientists posit that mergers could account for the asymmetry. I certainly don't see any remnants of a merger in this picture, though.

The radio image of the galaxy shows a strong jet that is likely due to interstellar medium interaction - this would lead to the conclusion that it's found near the disk plane. The radio feature is perhaps bending in an opposite direction due to precession. The radio jets are supposed to be from the centers of active galactic nuclei (AGN), so isn't it possible that the radio jet is precessing? Then it could be in the opposite direction to the optical features without a problem.

This certainly doesn't explain arms moving in opposite directions, though, as is seen in the example you gave. The simple explanation of 'it's due to a recent merger' seems to me a bit too vague. I wonder if anyone has gotten N-body simulations to show arms in opposite directions after throwing in some 'minor mergers'. I would still expect lots of starburst activity in those areas.

 

[TrickyDicky]

The particular galaxy I took a picture of has asymmetry in its spiral structure. Maybe it's due to a recent merger? I have read that some scientists posit that mergers could account for the asymmetry. I certainly don't see any remnants of a merger in this picture, though.

It's a reasonable possibility if you take for a moment the "crazy" view that I suggested earlier, that the arms so nicely depicted in your photo are actually the merger remnants in the form of tails in scape hyperbolic orbit away from the central core, insted of being in an elliptical rotating orbit around the center, just imagine that for a moment.