Formation of primordial galaxies under ultraviolet background radiation

The pancake collapse of pregalactic clouds under UV background radiation is explored with a one-dimensional sheet model. Here, attention is concentrat ed on elucidating the basic physics on the thermal evolution of pregalactic clouds exposed to diffuse UV radiation. So, we treat accurately the radiat ion transfer for the ionizing photons, with solving chemical reactions rega rding hydrogen molecules as well as atoms. The self-shielding against UV ra diation by H-2 Lyman-Werner bands, which regulates the photodissociation of hydrogen molecules, is also taken into account. As a result, it is found t hat when the UV background radiation is at a level of 10(-22) (nu/nu(L))(-1 ) ergs s(-1) cm(-2) Hz(-1) sr(-1), the cloud evolution bifurcates with st c ritical mass as M-SB = 2.2 x 10(11) M.[(1 + z(c))/5](-4.2), where z(c) is t he final collapse epoch. A cloud more massive than M-SB cools below 5 x 10( 3) K owing to H-2 line emission at the pancake collapse and would undergo t he initial starburst. The pancake possibly evolves into a virialized system in a dissipationless fashion. Consequently, this leads to the dissipationl ess galaxy formation at 3 less than or similar to z(c) less than or similar to 10. A cloud less massive than M-SB cannot cool by H-2 emission shortly after the pancake collapse but could cool in the course of shrinking to the rotation barrier. This is likely to lead to the dissipational galaxy forma tion at relatively low redshifts as 0 less than or similar to z(c) less tha n or similar to 4. The present results provide a solid physical mechanism t hat controls the star formation efficiency in the pregalactic clouds. In th e context of a standard CDM cosmology, M-SB lies between 1 sigma and 2 sigm a density fluctuations.