On star formation and the nonexistence of dark galaxies

We investigate whether a baryonic dark galaxy or "galaxy without stars'' could persist indefinitely in the local universe, while remaining stable against star formation. To this end, a simple model has been constructed to determine the equilibrium distribution and composition of a gaseous protogalactic disk. Specifically, we determine the amount of gas that will transit to a Toomre unstable cold phase via the H-2 cooling channel in the presence of a UV-X-ray cosmic background radiation field. All but one of the models are predicted to become unstable to star formation: we find that, in the absence of an internal radiation field, the majority of gas will become Toomre unstable in all putative dark galaxies with baryonic masses greater than 10(9) M circle dot, and in at least half of those greater than 10(6) M circle dot. Moreover, we find that all our model objects would be detectable via H I line emission, even in the case that star formation is potentially avoided. These results are consistent with the nondetection of isolated extragalactic H (I) clouds with no optical counterpart (galaxies without stars) by the H (I) Parkes All-Sky Survey. Additionally, where star formation is predicted to occur, we determine the minimum interstellar radiation field required to restore gravothermal stability, which we then relate to a minimum global star formation rate. This leads to the prediction of a previously undocumented relation between H (I) mass and star formation rate that is observed for a wide variety of dwarf galaxies in the H (I) mass range 10(8) - 10(10) M circle dot. The existence of such a relation strongly supports the notion that the well-observed population of dwarf galaxies represents the minimum rates of self-regulating star formation in the universe.