Star formation in disk galaxies driven by primordial H-2

We show that gaseous HI disks of primordial composition irradiated by an external radiation field can develop a multiphase medium with temperatures between 10(2) and 10(4) K as a result of the formation of molecular hydrogen. For a given HI column density there is a critical value of the radiation held below which only the cold HI phase can exist. Because of a time-decreasing quasar background, the gas starts cooling slowly after recombination until the lowest stable temperature in the warm phase is reached at a critical redshift z = z(alpha). Below this redshift the formation of molecular hydrogen promotes a rapid transition toward the cold HI phase. We find that disks of protogalaxies with 10(20) less than or similar to N-HI less than or similar to 10(21) cm(-2) are gravitationally stable at T similar to 10(4) K and can start their star formation history only at z less than or similar to z(cr) similar to 2, after the gas in the central portion of the disk has cooled to temperatures T less than or similar to 200 K. Such a delayed starbust phase in galaxies of low gas surface density and low dynamical mass can disrupt the disks and cause them to fade away. These objects could contribute significantly to the faint blue galaxy population.