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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 concentrated on elucidating the basic physics on the thermal evolution of pregalactic clouds exposed to diffuse UV radiation. So, we treat accurately the radiation transfer for the ionizing photons, with solving chemical reactions regarding hydrogen molecules as well as atoms. The self-shielding against UV radiation by H2 Lyman-Werner bands, which regulates the photodissociation of hydrogen molecules, is also taken into account. As a result, it is found that when the UV background radiation is at a level of 10-22(ν/νL)-1 ergs s-1 cm-2 Hz-1 sr-1, the cloud evolution bifurcates with a critical mass as MSB = 2.2 × 1011 M☉ -4.2, where zc is the final collapse epoch. A cloud more massive than MSB cools below 5 × 103 K owing to H2 line emission at the pancake collapse and would undergo the initial starburst. The pancake possibly evolves into a virialized system in a dissipationless fashion. Consequently, this leads to the dissipationless galaxy formation at 3 ≲ zc ≲ 10. A cloud less massive than MSB cannot cool by H2 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 formation at relatively low redshifts as 0 ≲ zc ≲ 4. The present results provide a solid physical mechanism that controls the star formation efficiency in the pregalactic clouds. In the context of a standard CDM cosmology, MSB lies between 1 σ and 2 σ density fluctuations.