Ultraviolet-background-induced bifurcation of galactic morphology

Based upon a novel paradigm of galaxy formation under the influence of an u ltraviolet background, the evolutionary bifurcation of pre-galactic clouds is compared with observations of elliptical and spiral galaxies. The theory predicts that the dichotomy between dissipational and dissipationless gala xy formation stems from the degree of self-shielding from the ultraviolet b ackground radiation. This is demonstrated on a bifurcation diagram of colla pse epochs versus masses of pre-galactic clouds. Using the observed propert ies, the collapse epochs and mass are assessed for each type of galaxy. By direct comparison of the theory with observations, it turns out that the th eoretical bifurcation branch successfully discriminates between elliptical and spiral galaxies. This suggests that the ultraviolet background radiatio n could play a profound role in the differentiation of galactic morphology into the Hubble sequence.