On the HI column density distribution of absorption systems of quasars

The observed distribution function of HI column densities in the absorption systems of quasars from NHI = 1013.5 cm-2 (Lyman α forest) through 1022 cm-2 (damped Lyman α systems) is well reproduced by considering the two-phase nature of intergalactic clouds confined by the gravity of cold dark matter, the so-called minihalos. By the shielding effect of diffuse UV flux due to neutral hydrogen, the optically thick neutral core appears in a massive minihalo; this core is detected as high column density system. The highly ionized outer envelope of a minihalo corresponds to a Lyman α forest. If star formation occurs in the collapsing core and ionizes the surrounding gas, this should be detectable as metal-line systems. In this way, we can have a unified picture of various absorption systems as minihalos with neutral cores. (author)