A Pressure Anomaly for HII Regions in Irregular Galaxies

Abstract

The pressures of giant HII regions in 6 dwarf Irregular galaxies are a factor of ~10 larger than the average pressures of the corresponding galaxy disks, obtained from the stellar and gaseous column densities. Either the visible HII 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 the possibility that the additional self-gravity comes from disk dark matter, but suggest this is unlikely because the vertical scale heights inferred for Irregular galaxies are consistent with the luminous matter alone. Some of the HII region overpressure is probably the result of local peaks in the gravitational field that come from large gas concentrations, many of which are observed directly. 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 HI phase as the first step toward dense molecular cores. Many of the HII regions could also be so strongly over-pressured that they will expand for a long time. In this case, the observed population would be only 7% of the total, and the aging HII regions, now too faint to see, should occupy nearly the entire dwarf galaxy volume. Such prolonged HII region expansion would explain the origin of the giant HI shells that are seen in these galaxies, and account for the lack of bright central clusters inside these shells.