22-February-2005, 05:45 AM
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Senior Member
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Join Date: Sep 2004
Posts: 145
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Quote:
In a sense, galaxy formation is like star formation (which we don't understand either)...
....So where did these perturbations come from? Statistical fluctuations in particle density within a uniform field won't do it. There are of order 1068 nucleons in a galaxy (neglecting dark matter, whose effects only make matters worse). This gives relative density fluctuations DN/N ~ 10-34, much too small for the most nonlinear growth to give galaxies by z=5.
....We might try considering primordial fluctuations (pushing our ignorance back as far as possible), but there is no signature of these in the microwave background
....We end up with one major problem: how could galaxies form so fast from the homogeneous background seen in the 2.7 K radiation?
....Longair devotes an entire chapter to setting out the "classical" scheme for galaxy formation in a purely baryonic Universe, solely to illustrate that it doesn't work.
....Hot dark matter makes galaxy formation very difficult - density enhancements are smeared away by rapid particle motions before they can collapse. The only unstable masses are far too large (remember MJ ~ T3/2 above). Cold dark matter is more popular, since this stuff is gravitationally bound and can fall into perturbations just as well as ordinary matter. However, it undergoes a fully dissipationless collapse. A problem here is that any dark matter cold enough to allow galaxy-size masses to form also predicts an uncomfortable amount of delayed galaxy formation. This gives difficulties with limits on baryonic IGM. There is not a clear piece of parameter space that simultaneously satisfies the IGM and microwave-background limits while still producing things of galactic mass. Plainly there is something important we don't know about galaxy formation.
....A key question must be answered from observation - what was the source of reionizing photons? One could imagine that either stars or active nuclei could provide the energy. Current surveys suggest that QSOs fall well short of the space density required to be important, although they will certainly dominate in their immediate neighborhoods. From metallicity arguments, first-generation stars were not numerous enough to ionize more than their immediate cloud vicinities. Further into galaxy buildup, we can count bright galaxies at redshifts z~6 by Lyman-break selection, and find that there are too few luminous galaxies for their hot stellar populations to power reionization
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