The impact of star formation on the interstellar medium in dwarf galaxies. II. The formation of galactic winds

Images and long-slit echelle spectra of the H alpha emission from 14 dwarf galaxies and M82 have been used to identify expanding shells of ionized gas. Supershells (radius >300 pc) are found in 12 of the dwarfs. The measured shell sizes and expansion speeds constrain the ages and power requirements of the bubbles. The dynamical age of the larger bubbles is typically about 10 Myr, and ionized shells older than 20 Myr are rare. An energy equivalent to 100-10,000 supernova explosions over this period is needed to drive the shock front that sweeps out the cavity. The current star formation rates are high enough to meet these power requirements. Many of the shells will break through the surrounding layer of H I supersonically, but the projected expansion speeds are typically less than the lower limits on the escape velocity. Some of the shell material may permanently escape from a few galaxies such as NGC 1569. Whether bound to the galaxy or not, these outflows probably play an important role in regulating the star formation rate and are expected to significantly influence the chemical evolution of the galaxies. The shells lift gas out of the disk at rates comparable to, or even greater than, the current galactic star formation rates. They will displace a substantial fraction of the interstellar gas only if their duty cycle is much longer than the rotational period of the disk.