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 21 cm line of neutral hydrogen (H I): 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 H I and deep optical observations of Holmberg I and M81 dw A, both members of the M81 group of galaxies. Their H I 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. If the neutral gas of Holmberg I was expelled by stellar winds and supernova explosions, resulting supposedly from a star formation event (or events) near the centre, the energy input is estimated to be similar to 10(53) erg. This translates to an upper limit for the age of this structure of similar to 9 x 10(7) years. As the HI column density towards the centre is about one order of magnitude lower than in the ring, a "blow-out" scenario is likely; however, the fact that the ring seems stationary argues against "blow-away", in which case material would be completely returned to the intergalactic environment. Optically, both galaxies fall clearly within the regime of low-surface brightness objects. Surface photometry based on Johnson UBVRI band observations shows an attenuation of brightness inside the H I-ring relative to an exponential disk model which fits the outer parts well. H alpha images of Holmberg I reveal a very low current star formation rate on the rim of the ring where HI column densities exceed 10(21) cm(-2).