Far‐Infrared Spectroscopy of Normal Galaxies: Physical Conditions in the Interstellar Medium

The most important cooling lines of the neutral interstellar medium (ISM) liein the far-infrared (FIR). We present measurements by the Infrared SpaceObservatory Long Wavelength Spectrometer of seven lines from neutral andionized ISM of 60 normal, star-forming galaxies. The galaxy sample spans arange in properties such as morphology, FIR colors (indicating dusttemperature), and FIR/Blue ratios (indicating star-formation activity andoptical depth). In two-thirds of the galaxies in this sample, the [CII] line isproportional to FIR dust continuum. The other one-third show a smooth declinein [CII]/FIR with increasing F60/F100 and FIR/B, spanning a range of a factorof more than 50. Two galaxies, at the warm and active extreme of the range have[CII]/FIR < 2 \times 10^{-4} (3-sigma upper limit). This is due to increasedpositive grain charge in the warmer and more active galaxies, which leads toless efficient heating by photoelectrons from dust grains. The ratio of the twoprincipal photodissociation region (PDR) cooling lines [CII]/[OI] shows a tightcorrelation with F60/F100, indicating that both gas and dust temperaturesincrease together. We derive a theoretical scaling between [NII] and [CII] fromionized gas and use it to separate [CII] emission from neutral PDRs and ionizedgas. Comparison of PDR models of Kaufman et al. (1999) with observed ratios of(a) [OI]/[CII] and ([CII]+[OI])/FIR and (b) [OI]/FIR and F60/F100 yields far-UVflux G0 and gas density n. The derived G0 scales as n to the power 1.4. Weinterpret this correlation as arising from Stromgren sphere scalings if much ofthe line and continuum luminosity arises near star-forming regions. Thedifferences in G0 and n may be due to differences in the physical properties ofthe star-forming clouds.(Short abstract)Comment: Accepted by the Astrophysical Journa