The Ionized Gas in Local Starburst Galaxies: Global and Small-Scale Feedback from Star Formation

Abridged: The small-- and intermediate--scale structure and the fraction ofthe ISM ionized by non--radiative processes is investigated in a small sampleof four local starburst galaxies, imaged with the HST WFPC2. The samplecomprises three dwarf galaxies, NGC3077, NGC4214, and NGC5253, and one giantspiral, NGC5236 (M83). The galaxies span a range in metallicity, luminosity,and environment, enabling the investigation of non--radiative ionizationprocesses in a variety of galactic conditions. For this purpose, the fourgalaxies were imaged in the lines of H-beta(4861 A), [OIII](5007 A),H-alpha(6563 A), and [SII](6717,6731 A). The non--photoionized gas in thesegalaxies has been traced on scales ranging from a few tens of pc to a fewhundred pc, providing a full budget for this ionized gas component. Using the`maximum starburst line' of Kewley et al. (2001) to discriminate betweenphotoionized and non--photoionized gas, we find that in all four galaxiesnon--photoionization processes are responsible for a small fraction of thetotal H-alpha emission, at the level of 3%--4%. The starbursts in the threedwarf galaxies deposit a significant fraction, 70%-100%, of their mechanicalenergy into the surrounding interstellar medium and require time--extended (afew x 10^7 yr to ~10^8 yr) star formation, in order to account for the observedluminosity of the non--photoionized gas. In the massive spiral, thenon--photoionized gas is concentrated in localized areas surrounded by activestar--formation, with no evidence for extended structures (as those in thedwarfs). This confirms the picture that starbursts remain confined events inmassive galaxies, likely due to the deep potential well