  We perform radiation hydrodynamics simulations on the evolution of galactic
gas disks irradiated by ultraviolet radiation background. We find gas disks
with N_H > 10^21 cm^-2 exposed to ultraviolet radiation at a level of I_21=1
can be self-shielded from photoheating, whereas the disk with N_H < 10^21 cm^-2
cannot. We also find that the unshielded disks keep smooth density distribution
without any sign of fragmentation, while the self-shielded disks easily
fragment into small pieces by self-gravity, possibly followed by star
formation. The suppression of star formation in unshielded disks is different
from photoevaporation effect, since the assumed dark halo potential is deep
enough to keep the photoheated gas. Presence of such critical threshold column
density would be one of the reason for the so-called down-sizing feature of
present-day galaxies.
