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Galaxies without dark matter halos?
[[Mod. note -- I have taken the liberty of removing a crosspost to
sci.physics.research, as (IMHO) this article falls reasonably within s.a.r's subject area (astronomy/astrophysic) but is a bit off-topic for s.p.r. -- jt]] Joseph Lazio wrote in message ... 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. The fact remains that in most publications gas rotation curves are used to support the hypothesis that stars are bound by dark matter. Why ? Presumably because star rotation curves wouldn't be as conclusive. 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, outside the optical disk you would be hard pressed to explain the rotation curves by low mass stars, but here, as you admitted above, the curves do only represent the gas motion which could be affected by a rotating magnetic field even in its neutral form (as explained on my webpage http://www.physicsmyths.org.uk/darkmatter.htm ). However, even if observations of stars and globular clusters seem to indicate the presence of dark matter, this is in my opinion not a foregone conclusion, as the situation is by no means comparable for instance to the discovery of Neptune (as claimed in the sci.astro FAQs): in the latter case the conclusion could be reached solely by position measurements, whereas for the galactic problem you have to make additional assumptions regards the absolute brightness of the objects. Any error in the mass-luminosity relationship could affect the conclusion crucially. If you have a look for instance at http://planetquest.jpl.nasa.gov/SIM/science_henry.pdf , you will find that the luminosities for stars less than 1 solar mass are indeed uncertain by about 2-3 magnitudes. I find it quite remarkable that the mass luminosity relationship, which a) is quite uncertain for low mass stars, b) obtained only in the solar neighbourhood and c) obtained only from double stars, is applied to all stars in our or other galaxies regardless. I don't think that all these points have been sufficiently examined to justify the conclusion of dark matter from the dynamics of galactic objects. Besides, in the case of Uranus/Neptune, the 'dark matter' was identified after a very short time, but decades of research have been unable to clarify the physical nature of the galactic 'dark matter'. |
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