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 hydrogen has recombined, as the external ionizing UV flux decreases and the metal abundance Z increases, the H I, initially in the warm phase (T ≳ 5000 K), makes a transition to the cool phase (T ≲ 100 K). The minimum abundance Zmin for which this phase transition takes place in a small fraction of the Hubble time decreases rapidly with increasing gas column density. Therefore, in "anemic" disk galaxies where NH I is up to 10 times smaller than for normal large spirals, the onset of the cool H I phase is delayed. The onset of gravitational instability is also delayed since these objects are more likely to be gravitationally stable in the warm phase than progenitors of today's large spiral galaxies. The first substantial burst of star formation may occur only as late as redshift z ∼ 0.5 and may give a temporary high peak luminosity, possibly related to the "faint blue objects." Galaxy disks of lower column density tend to have lower escape velocities and a starburst/galactic fountain instability that decreases the gas content of the inner disk drastically.