A Phase Change scenario for Galaxy Formation
The 5 independent correlations between the properties of galaxies observed outside big clusters must set such strong constraints on any theory of galaxy formation that it is hard to imagine any but the right one passing muster; certainly the standard model of Hierarchical Formation does not. Furthermore those global correlations imply that such galaxies must be remarkably simple, i.e. have virtually all of their variance attributable to a single Principal Component. That being so the correlations ought to give strong hints as to the process of formation. Two in particular: all galaxies have the same Luminosity-Density, while Luminosity and Dynamical Mass are everywhere tightly correlated, hint that formation took place at the epoch of Recombination. Halving of particle numbers then will set up strong pressure-forces which could fragment the medium and halt the expansion of neutral fragments, if they are not too large. We find that fragments between $10^{11} $ and $10^{6} $ solar masses (baryonic) will collapse gravitationally upon themselves, mostly flattening in one dimension, to form galaxies between 10 kpc. and 0.3 kpc. in size, and in two distinct classes. Those below $10^{8}$ solar masses will remain transparent and cool, eventually sub-fragmenting into stars with a warm, spheroidal distribution, while those above will become opaque and thus too hot to form stars until after they have radiated away most of their axial binding energy. They will then settle into thin discs with a low axial velocity dispersion. This simple scenario generates most of the properties of the galaxies but not flat rotation-curves. Finally we attempt to weigh up its pros and cons as a viable theory of galaxy formation and conclude that it is much superior to the Hierarchical model when it comes to fitting most observations. At the very least it deserves further critical evaluation.