Small Galaxies Blowing Big Bubbles

Some of the lowest-mass dwarf irregular galaxies, such as Leo A, Sag DIG and Cas 1, show a characteristic distribution of their neutral interstellar medium (ISM) as observed in the 21cm line of neutral hydrogen (HI): the majority of the gas is found to be concentrated in a ring-like feature centred on, and embracing the optical counterparts of these objects. We present multi-array, high-resolution VLA HI and deep optical observations of Holmberg I and M81 dw A, both members of the M81 group of galaxies. Their HI is mainly stored at a galactocentric radius of about 0.8 kpc, at the edge of the optical images. The rings show neither expansion nor contraction.