THE ONSET OF THE COLD H-I PHASE IN DISKS OF PROTOGALAXIES

We discuss a possible delay experienced by protogalaxies with low gas column densities in forming stars over large scales. After the hydroge n has recombined, as the external ionizing UV flux decreases and the m etal abundance Z increases, the H I, initially in the warm phase (T gr eater than or similar to 5000 K), makes a transition to the cool phase (T less than or similar to 100 K). The minimum abundance Z(min) for w hich this phase transition takes place in a small fraction of the Hubb le time decreases rapidly with increasing gas column density. Therefor e, in ''anemic'' disk galaxies, where N-HI is up to 10 times smaller t han for normal large spirals, the onset of the cool H I phase is delay ed. The onset of gravitational instability is also delayed since these objects are more likely to be gravitationally stable in the warm phas e than progenitors of today's large spiral galaxies. The first substan tial burst of star formation may occur only as late as redshift z simi lar to 0.5 and may give a temporary high peak luminosity, possibly rel ated to the ''faint blue objects.'' Galaxy disks of lower column densi ty tend to have lower escape velocities and a starburst/galactic fount ain instability that decreases the gas content of the inner disk drast ically.