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We report measurements of the C II fine-structure line at 157.714 μm in 30 normal star-forming galaxies with the Long Wavelength Spectrometer (LWS) on the Infrared Space Observatory (ISO). The ratio of the line to total far-infrared (FIR) luminosity, LC II/LFIR, measures the ratio of the cooling of gas to that of dust, and thus the efficiency of the grain photoelectric heating process. This ratio varies by more than a factor of 40 in the current sample. About two-thirds of the galaxies have LC II/LFIR ratios in the narrow range of (2-7) × 10−3. The other one-third show trends of decreasing LC II/LFIR with increasing dust temperature, as measured by the flux ratio of infrared emission at 60 and 100 μm, Fν(60 μm)/Fν(100 μm), and with increasing star formation activity, measured by the ratio of FIR and blue-band luminosity, LFIR/LB. We also find three FIR-bright galaxies that are deficient in the C II line, which is undetected with 3 σ upper limits of LC II/LFIR < (0.5-2) × 10−4.
Malhotra et al. (Wed,) studied this question.