A pressure anomaly for H II regions in irregular galaxies

Abstract

The pressures of giant H II regions in six dwarf irregular galaxies are found to be a factor of ∼10 larger than the average pressures of the corresponding galaxy disks, obtained from the stellar and gaseous column densities. This is unlike the situation for spiral galaxies, where these two pressures are approximately equal. Either the H II regions in these dwarfs are all so young that they are still expanding, or there is an unexpected source of disk self-gravity that increases the background pressure. We consider first whether any additional self-gravity might come from disk dark matter that either is cold H2 gas in diffuse or self-gravitating clouds with weak CO emission, or is the same material as the halo dark matter inferred from rotation curves. The H2 solution is possible because cold molecular clouds would be virtually invisible in existing surveys if they were also CO-weak from the low metal abundances in these galaxies. Cosmological dark matter might be possible too because of the relatively large volume fraction occupied by the disk within the overall galaxy potential. There is a problem with both of these solutions, however: the vertical scale heights inferred for irregular galaxies are consistent with the luminous matter alone. The amount of disk dark matter that is required to explain the high H H region pressures would give gas and stellar scale heights that are too small. The anomalous pressures in star-forming regions are more likely the result of local peaks in the gravitational field that come from large gas concentrations. These peaks also explain the anomalously low average column density thresholds for star formation that were found earlier for irregular galaxies, and they permit the existence of a cool H I phase as the first step toward dense molecular cores. The evidence for concentrations of H I in regions of star formation is summarized; the peak column densities are shown to be consistent with local pressure equilibrium for the H II regions. Strongly self-gravitating star-forming regions should also limit the dispersal of metals into the intergalactic medium. The third possibility is that all of the visible H II regions in these dwarf galaxies are strongly overpressured and still expanding. The mean time to pressure equilibrium is ∼15 times their current age, which implies that the observed population is only 7% of the total if they live that long; the rest are presumably too faint to see. The expansion model also implies that the volume-filling factor can reach ∼100 times the current factor, in which case faint and aging H II regions should merge and occupy nearly the entire dwarf galaxy volume. This would explain the origin of the giant H I shells seen in these galaxies as the result of old, expanded H II regions that were formerly driven by OB associations. The exciting clusters would now be so old and dispersed that they would not be recognized easily. The shells are still round because of a lack of shear.