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 H2 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 109 M(circled dot opertaor), and in at least half of those greater than 106 M(circled dot opertaor). 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(circled dot operator). 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.