Formation of Primordial Galaxies under Ultraviolet Background Radiation
Susa, Hajime, & Umemura, Masayuki
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 H$_2$ 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$^-22$(\ensuremathν/\ensuremathν$_L$)$^-1$ ergs s$^-1$ cm$^-2$ Hz$^-1$ sr$^-1$, the cloud evolution bifurcates with a critical mass as M$_SB$=2.2×10$^11$ M$_solar$[(1+z$_c$)/5]$^-4.2$, where z$_c$ is the final collapse epoch. A cloud more massive than M$_SB$ cools below 5×10$^3$ K owing to H$_2$ 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<åisebox-0.5ex\text asciitildez$_c$<\i̊sebox-0.5ex~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 formation at relatively low redshifts as 0<\rs̊ebox-0.5ex~z$_c$<\rae̊box- 0.5ex~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, M$_SB$ lies between 1 \ensuremathσ and 2 \ensuremathσ density fluctuations.