OBSERVATIONS OF THE IMPACT OF STARBURSTS ON THE INTERSTELLAR-MEDIUM IN DWARF GALAXIES

Dwarf galaxies play a crucial role in our understanding of the formati on and evolution of galaxies, and the concept of supernova-driven mass outflows is a vital ingredient in theories of the structure and evolu tion of dwarf galaxies. Despite the theoretical importance of these ou tflows, there is a very limited amount of direct observational evidenc e for their existence. We have therefore begun a detailed multi-wave-b and search for outflows in dwarf (M(B) greater than or equal to -18) g alaxies with extensive recent or ongoing centrally concentrated star f ormation. We report the first results of this search in the present pa per. Observations of the ionized gas in dwarf amorphous galaxies with centrally concentrated populations of massive stars provide evidence f or the large-scale expansion of their ionized interstellar media. Fabr y-Perot H alpha images reveal the presence of kiloparsec-scale ''super bubbles'' and filaments which tend to be oriented along the galaxy min or axis. These structures are comparable in size to the characteristic optical sizes of the galaxies, and dominate the morphology of the gal axies at low surface brightness in H alpha. Echelle spectra taken thro ugh these structures demonstrate that the kinematics are consistent wi th hollow kiloparsec-scale structures expanding at typical velocities of about 50 km s(-1) (ranging from about 25 and 100 km s(-1)). The mec hanical energy output from the supernovae and stellar winds in the cen tral region of each galaxy appears adequate to power expansion motions of this size and speed. Since the inferred expansion velocities are c omparable to (but probably a bit smaller than) the rather uncertain es cape velocities from these galaxies, the ultimate fate of the accelera ted gas is not clear. However, even if this relatively cool and dense H alpha-emitting gas is not ejected from the galaxies, it is likely th at the much hotter (supernova and stellar wind heated) gas in the inte rior of the superbubble will escape the galaxies once the superbubble fragments. Since this hot gas contains the newly synthesized metals, t he chemical evolution of dwarf galaxies may be strongly affected by th ese outflows, even if the galaxy is able to retain most of its interst ellar medium. Since expanding structures of this size and velocity are not observed in all low-mass galaxies with recent or ongoing star for mation, we suggest that we are witnessing transient events that likely have a relatively low ''duty cycle'' in such galaxies. That is, we ar gue that the particular galaxies in the present paper have had signifi cantly elevated star formation rates over the past 10(7)-10(8) yr (i.e ., these are starburst or young poststarburst systems). This interpret ation is consistent with the optical colors and emission-line properti es of these galaxies.