Type: Article
Publication Date: 2004-02-27
Citations: 83
DOI: https://doi.org/10.1086/382095
The small- and intermediate-scale structure and the fraction of the interstellar medium (ISM) ionized by nonradiative processes are investigated in a small sample of four local starburst galaxies, imaged with the Hubble Space Telescope (HST) Wide Field Planetary Camera 2. The sample comprises three dwarf galaxies, NGC 3077, 4214, and 5253, and one giant spiral, NGC 5236 (M83). The galaxies span a range in metallicity (∼0.2–2 Z⊙), luminosity (MB ∼ -17 to -20), and environment (isolated, interacting), enabling the investigation of nonradiative ionization processes in a variety of galactic conditions. For this purpose, the four galaxies were imaged in the lines of Hβ (4861 Å), [O III] (5007 Å), Hα (6563 Å), and [S II] (6717, 6731 Å). This is a unique set of data, as very few galaxies (and only our four starbursts) have ever been imaged by HST in the relatively faint lines of Hβ and [S II]. The use of HST has allowed us to trace nonphotoionized gas in these galaxies on scales ranging from a few tens of parsecs to a few hundred parsecs and, thus, to provide a full budget for this ionized gas component. Using the "maximum starburst line" of Kewley et al. to discriminate between photoionized and nonphotoionized gas, we find that in all four galaxies non-photoionization processes are responsible for a small fraction of the total Hα emission, at the level of 3%–4%. Because the nonphotoionized gas is associated with low Hα surface brightness, it occupies between one-sixth and one-quarter of the total imaged area. The central starbursts yield enough mechanical energy to produce the nonphotoionized gas in the four galaxies, by means of shocks from massive star winds and supernova explosions. In particular, the starbursts in the three dwarf galaxies must deposit a significant fraction, 70%–100%, of their mechanical energy into the surrounding ISM, in order to account for the observed luminosity of the nonphotoionized gas. The morphology of the nonphotoionized regions is different in the dwarfs and the giant spiral. As already established in previous works, nonphotoionized gas in dwarfs is mainly associated with extended "shells" or filamentary regions, likely areas of supernova-driven expanding gas. In all three dwarfs, star formation has been an ongoing process for the last few times 107 yr to ∼108 yr; time-extended star formation episodes are a requirement to sustain the observed luminosity of the nonphotoionized gas. In the massive spiral, the nonphotoionized gas is concentrated in localized areas surrounded by active star formation, with no evidence of extended structures on the same (or smaller) spatial scales as the "shells" in the dwarfs. The two Hα cavities in NGC 5236 may be evolved regions within the starburst. This confirms the picture that starbursts remain confined events in massive galaxies, likely as a consequence of the deep potential well.