The Onset of the Cold H i Phase in Disks of Protogalaxies

We discuss a possible delay experienced by protogalaxies with low column density of gas 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 HI, initially in the warm phase ($Tsimgt 5000$ K), makes a transition to the cool phase ($Tsimlt 100$ K). The minimum abundance $Z_{min}$ for which this phase transition takes place in a small fraction of the Hubble time decreases rapidly with increasing gas column density. Therefore in the ``anemic'' disk galaxies, where $N_{HI}$ is up to ten times smaller than for normal large spirals, the onset of the cool HI 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 at redshifts $z sim 0.5$ and give a temporary high peak luminosity, which may be related to the ``faint blue objects". Galaxy disks of lower column density tend to have lower escape velocities and a starburst/galactic fountain instability which decreases the gas content of the inner disk drastically.Comment: TeX file, 24 pages, 4 figures available upon request from edvige@arcetri.astro.it, to appear in The Astrophysical J. (Sept. 1