  We argue that the lack of observed damped Lyman alpha (DLA) systems that
simultaneously have high HI columns densities and high metallicities results
naturally from the formation of molecules in the cold phase of a two-phase
atomic medium in pressure balance. Our result applies equally well in diffuse
systems where the ultraviolet radiation field is dominated by the extragalactic
background and in dense star-forming ones, where the local radiation field is
likely to be orders of magnitude higher. We point out that such a
radiation-insensitive model is required to explain the absence of high column -
high metallicity systems among DLAs observed using gamma-ray burst afterglows,
since these are likely subjected to strong radiation fields created by active
star formation in the GRB host galaxy. Moreover, we show that the observed
relationship between the maximum atomic gas column in DLAs sets a firm upper
limit on the fraction of the mass in these systems that can be in the warm,
diffuse phase. Finally, we argue that our result explains the observed lack of
in situ star formation in DLA systems.
