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Galaxies without dark matter halos?
"TS" == Thomas Smid writes:
TS Joseph Lazio wrote in message TS ... Vega-Beltran et al. (1999, Ap&SS) find, "In the outer regions of all of the sample of 20 galaxies the stellar rotation is comparable to that of the ionized gas [...], while in the inner regions the following kinematic features are noteworthy." TS I had a closer look at some of Vega-Beltran's papers now as well TS and noticed that these studies regards the difference between star TS and gas velocities are confined to some 'early-type' spirals, and TS it is quite possible that for these the magnetic dynamo may not TS have fully developed yet, hence explaining the co-rotation of the TS gas and the stars in the outer regions, [...] Notice that Vega-Beltran's work concerns *ionized* gas. Ionized gas should be quite receptive to any magnetic influences. Indeed, most people assume that ionized gas is essential in producing a magnetic dynamo. TS look at any textbook or webpage about galaxies and you will only TS find rotation curves based on gas (usually HI 21cm measurements) TS and the co-rotation of stars assumed implicitly without any TS justification [...] You're mixing up two different measurements. Within the optical disk, I think gas and stars usually co-rotate. (There is a small number of important exceptions, but the general rule is co-rotation.) Outside the optical disk, no stars are detected so one has to rely on the gas to trace the gravitational potential. TS Anyway, I wonder how one can be so sure about the exact amount and TS distribution of the 'visible mass' in a galaxy and therefore the TS need for dark matter in the first place: I can't imagine that the TS stars have been literally counted, so maybe there are actually TS more stars than assumed (according to the mass-luminosity TS relationship, a star with half the mass has only 1/10 of the TS luminosity , so with 10 times as many stars of half the mass, you TS would have the same overall brightness but 5 times the overall TS mass, which might dispense with the need for dark matter). So work out the numbers in more detail; I'd actually be curious to see this. Take a couple of example late-type spirals for which good rotation curves exist. Assume that all of the dark matter is in the form of 0.1 solar mass stars. How many stars would be required and would the integrated light from them still be undetectable? Of course, you also have the related problem of, How would those stars get there? Star formation in spiral galaxies falls off quite quickly at large distances from the center of the galaxies, and the gas in the outer parts of spiral galaxies is fairly diffuse. How would stars form out there? -- Lt. Lazio, HTML police | e-mail: No means no, stop rape. | http://patriot.net/%7Ejlazio/ sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html |
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