UV Background-Induced Bifurcation of the Galactic Morphology

Based upon a novel paradigm of the galaxy formation under UV background, the evolutionary bifurcation of pregalactic clouds is confronted with observations on elliptical and spiral galaxies. The theory predicts that the dichotomy between the dissipational and dissipationless galaxy formation stems from the degree of self-shielding from the UV background radiation. This is demonstrated on a bifurcation diagram of collapse epochs versus masses of pregalactic clouds. Using the observed properties, the collapse epochs are assessed for each type of galaxies with attentive mass estimation. By the direct comparison of the theory with the observations, it turns out that the theoretical bifurcation branch successfully discriminates between elliptical and spiral galaxies. This suggests that the UV background radiation could play a profound role for the differentiation of the galactic morphology into the Hubble sequence.