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

The most important cooling lines of the neutral interstellar medium (ISM) lie in the far-infrared (FIR). We present measurements by the Infrared Space Observatory Long Wavelength Spectrometer of seven lines from neutral and ionized ISM of 60 normal, star-forming galaxies. The galaxy sample spans a range in properties such as morphology, FIR colors (indicating dust temperature), and FIR/blue ratios (indicating star formation activity and optical depth). In two-thirds of the galaxies in this sample, the [C II] line flux is proportional to FIR dust continuum. The other one-third show a smooth decline in L[C II]/L_FIR with increasing F_ν(60 μm)/F_ν(100 μm) and L_FIR/L_B, spanning a range of a factor of more than 50. Two galaxies at the warm and active extreme of the range have L[C II]/L_FIR < 2 × 10^-4 (3 σ upper limit). This is due to increased positive grain charge in the warmer and more active galaxies, which leads to less efficient heating by photoelectrons from dust grains. The ratio of the two principal photodissociation region (PDR) cooling lines L[O I]/L[C II] shows a tight correlation with F_ν(60 μm)/F_ν(100 μm), indicating that both gas and dust temperatures increase together. We derive a theoretical scaling between [N II] (122 μm) and [C II] from ionized gas and use it to separate [C II] emission from neutral PDRs and ionized gas. Comparison of PDR models of Kaufman et al. with observed ratios of (1) L[O I]/L[C II] and (L[C II] + L[O I])/L_FIR and (2) L[O I]/L_FIR and F_ν(60 μm)/F_ν(100 μm) yields far-UV flux G₀ and gas density n. The G₀ and n values estimated from the two methods agree to better than a factor of 2 and 1.5, respectively, in more than half the sources. The derived G₀ and n correlate with each other, and G₀ increases with n as G₀ ∝ n^α, where α ≈ 1.4 . We interpret this correlation as arising from Strömgren sphere scalings if much of the line and continuum luminosity arises near star-forming regions. The high values of PDR surface temperature (270-900 K) and pressure (6 × 10⁴-1.5 × 10⁷ K cm^-3) derived also support the view that a significant part of grain and gas heating in the galaxies occurs very close to star-forming regions. The differences in G₀ and n from galaxy to galaxy may be due to differences in the physical properties of the star-forming clouds. Galaxies with higher G₀ and n have larger and/or denser star-forming clouds.