Properties of ionized interstellar froth in irregular galaxies

Abstract

Results are reported from new observations of large-scale H-alpha filamentary structures lying outside of classical H II regions in Magellanic irregular galaxies. These structures ('froth') are seen as arcs, loops, and filaments, some of which are centered on OB associations and some of which extend away from giant H II regions into regions of lower H I column density. The data show the following: (1) the level of ionization of the froth is lower than in H II regions in the same galaxies; (2) frothy features have large line velocity widths that often exceed those found in classical giant H II regions in the same galaxies; (3) the froth has H-alpha surface brightnesses of about 100 Rayleighs, 10-100 times brighter than typical diffuse Galactic emission; and (4) dimensions are large, a typical filament is 700 x 50 pc.