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TrickyDicky
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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 said: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.
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.htmlkungfuscious said: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.
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 said:The dark matter is inferred by the flatness of the rotation curves.
kungfuscious said: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
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.kungfuscious said: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.
Spiral rotation in disk galaxies refers to the swirling movement of stars and gas within the disk of a galaxy. This rotation is caused by the gravitational pull of the galaxy's central bulge, and it is responsible for the distinctive spiral arms seen in many disk galaxies.
Spiral rotation in disk galaxies is studied through a combination of observations and theoretical models. Observations, such as those from telescopes and satellites, provide data on the motion and distribution of stars and gas in galaxies. Theoretical models use this data to make predictions about the physical processes that drive spiral rotation.
The physical basis of inference in studying spiral rotation is the use of known physical laws and theories to interpret observations and make predictions. By understanding the underlying physical processes that drive spiral rotation, scientists can make inferences about the structure and evolution of disk galaxies.
Current areas of research in spiral rotation in disk galaxies include studying the role of dark matter in shaping galaxy rotation curves, investigating the effects of interactions and mergers on spiral galaxies, and understanding the relationship between spiral structure and the overall evolution of galaxies.
The study of spiral rotation in disk galaxies has numerous potential applications, such as improving our understanding of the formation and evolution of galaxies, providing insights into the distribution and properties of dark matter, and helping us better understand the dynamics of the universe as a whole. Additionally, this research can have practical applications in fields such as astrophysics, cosmology, and space exploration.