Effects of massive star formation on the ISM of dwarf galaxies

We are studying star. formation effects on the properties of the ISM in low metallicity environments using mid-infrared (MIR) and far-infrared (FIR) o bservations of starbursting dwarf galaxies taken with the Infrared Space Ob servatory (ISO) and the Kuiper Airborne Observatory (KAO), Effects of the h ard pervasive radiation field on the gas and dust due to the dust-pool envi ronments are apparent in both the dust and gas components. From a 158 mum [ CII] survey we find enhanced I[CII]/FIR ratios in dwarf galaxies acid I[CII ]/I(CO) ratios up to 10 times higher than those for normal metallicity star burst galaxies. We consider MIR observations in understanding the star form ation properties of dwarf galaxies and constraints on the stellar SED. Nota bly, the strong MIR [NeIII]/[NeII] ratios reveal the presence of current ma ssive stellar populations <5 My old in NGC 1569, NGC 1140 and IIZw40. The M IR unidentified infrared bands (UIBs) are weak, if present at all, as a gen eral characteristic in low metallicity environments, revealing the destruct ion of the smallest carbon particles (e.g. PAHs) over large spatial scales. This is confirmed with our dust modeling: mass fractions of PAHs are almos t negligible compared to the larger silicate grains emitting in the FIR as well as the small carbon grains emitting in the MIR, which appear to be the source of the photoelectric gas heating in these galaxies, in view of the [CII] cooling. (C) 2000 Elsevier Science B.V. All rights reserved.