Using 3D radiative transfer calculations on the reionization of an inhomogeneous universe , QSO absorption line systems are simulated and they are compared with observations of Ly \alpha continuum depression at high redshifts .
By this comparison , it is found that the metagalactic UV intensity decreases rapidly with z at z > 4 as I _ { 21 } = 0.5 \exp [ 3 ( 4 - z ) ] , and the reionization must have taken place between z = 6 and 10 .
Based on this time-dependence of UV background intensity , we explore the collapse of pregalactic clouds in the UV background , and find that the self-shielding is prominent above a mass scale as M _ { BIF } = 3.0 \times 10 ^ { 11 } M _ { \odot } [ ( 1 + z _ { c } ) / 5 ] ^ { -4.2 } ( I _ { 21 } / 0.5 ) ^ { 0.6 } . This mass scale predicts the bifurcation of galactic morphology , and by confrontation with observations it turns out that the bifurcation mass successfully discriminates between elliptical and spiral galaxies .