Pervasive Cavity-rim Structure for Star Formation in Dwarf Irregular Galaxies

Abstract

Unsharp-mask images of H I emission from 36 dwarf irregular (dIrr) galaxies illustrate star formation in dispersed clouds and on the rims of large cavities. The cavities can extend for a radial scale length and typically have circular or slightly sheared forms. The average surface density of cloud peaks is ∼20 M⊙ pc−2, and, combined with their average far-UV star formation rate, this suggests a gas consumption time of ∼3.2 Gyr. Vertical hydrostatic equilibrium calculations for 24 of these dIrrs give a typical scale height of ∼400 pc, which combines with the gas and star formation surface densities to suggest an efficiency per freefall time of ∼1%. These values are comparable to those in the molecular clouds of spiral galaxies, suggesting that the primary difference between clouds is the presence of CO at higher metallicity in the spirals. U − B color images of the dIrrs suggest that cavity ages range between 107 and 108 yr, with the longer times explaining the common lack of bright OB associations in their centers and their low expansion speeds. Most are circular because the shear time exceeds 100 Myr, although some of the H I has spiral structure. These observations suggest that star formation in dIrrs proceeds slowly in a sequential fashion in dispersed clouds and on the periphery of giant cavities that move and expand during the ∼50 Myr supernova era of the previous generation. In contrast, spiral galaxies have shear times 10× shorter and more important stellar dynamics that compresses the gas into filaments.