A Pressure Anomaly for HiiRegions in Irregular Galaxies

The pressures of giant HII regions in 6 dwarf Irregular galaxies are a factorof ~10 larger than the average pressures of the corresponding galaxy disks,obtained from the stellar and gaseous column densities. Either the visible HIIregions in these dwarfs are all so young that they are still expanding, orthere is an unexpected source of disk self-gravity that increases thebackground pressure. We consider the possibility that the additionalself-gravity comes from disk dark matter, but suggest this is unlikely becausethe vertical scale heights inferred for Irregular galaxies are consistent withthe luminous matter alone. Some of the HII region overpressure is probably theresult of local peaks in the gravitational field that come from large gasconcentrations, many of which are observed directly. These peaks also explainthe anomalously low average column density thresholds for star formation thatwere found earlier for Irregular galaxies, and they permit the existence of acool HI phase as the first step toward dense molecular cores. Many of the HIIregions could also be so strongly over-pressured that they will expand for along 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 theentire dwarf galaxy volume. Such prolonged HII region expansion would explainthe origin of the giant HI shells that are seen in these galaxies, and accountfor the lack of bright central clusters inside these shells.Comment: 27 pages, 4 figures, Astrophysical Journal, 540, Sep 10, 2000, in pres