What is the evidence for the density wave theory for spiral galaxies? I've heard that it has a hard time explaining the very detailed spiral structure in the central hub of the Whirlpool galaxy, M51. The spiral arms are wrapped about the center for about three full turns, which the density wave model does not explain well. Kennedy sums up the problem: "…the precise physical recipe that predicts their [density waves’] behaviour continues to elude us".
I read about another problem on Kash Farooq's blog:
"If star formation occurs when the faster differentially rotating gas clouds hits the spiral density wave, then there should be some sort of spatial ordering of objects in the arms. Very young objects should be observed at one edge, objects that are a bit older would be found in the middle and the oldest objects would be observed at the opposite edge of the arm.
And this is something that is testable.
And here is a paper that looks for that evidence: “Observational Evidence Against Long-Lived Spiral Arms in Galaxies” by Foyle et al. (25 May 2011).
The authors looked at 12 nearby spiral galaxies. They failed to find such young-to-old ordering. They conclude:
'…result indicates that spiral density waves in their simplest form are not an important aspect of explaining spirals in large disk galaxies.' "
I've also heard that galaxies such as NGC 6872, UGC 10214, and NGC 4676 are difficult to model as a density wave phenomenom. In addition flocculent spirals such as NGC 4414 have spiral structure that do not result from density waves. Then there is the blue compact dwarf galaxy NGC 2915 which has an extended hydrogen disk that forms a nice barred spiral.
Some other problems have also been pointed out by Halton Arp.
"Nevertheless, the density-wave theory has been very successful in explaining the general structure of galaxies such as M81 and M74, as well as the Milky Way. Analysis of the motions of gas in M81 has also confirmed the predictions of the density-wave theory. In fact, despite some difficulties with the details of M51, this galaxy, too, has confirmed the more general predictions of the density-wave theory."
 Kenney, J., "More whirls in the whirlpool," Nature 364:283–284, 1993.
 Kash Farooq, "Spiral Galaxies, Density Waves Theory and Dark Matter," The Thought Stash (April 18, 2012); http://thethoughtsta...nd-dark-matter/
 Arp, Halton (Dec. 1986), "The persistent problem of spiral galaxies," IEEE Transactions on Plasma Science (ISSN 0093-3813), vol. PS-14, p. 748-762; http://adsabs.harvar...3e5c03c80a10724
 Justin K. Taylor (2005), "The speed of matter," TJ 19(3):94; http://creation.com/...j19_3_91-95.pdf
Density Wave Theory
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