INFRARED SPACE-OBSERVATORY MEASUREMENTS OF [C-II] LINE VARIATIONS IN GALAXIES

We report measurements of the [C II] fine-structure line at 157.714 mu m in 30 normal star-forming galaxies with the Long Wavelength Spectro meter (LWS) on the Infrared Space Observatory, (ISO). The ratio of the line to total far-infrared (FIR) luminosity, L-[CII]/L-FIR, measures the ratio of the cooling of gas to that of dust, and thus the efficien cy of the grain photoelectric heating process. This ratio varies by mo re than a factor of 40 in the current sample. About two-thirds of the galaxies have L-[CII]/L-FIR ratios in the narrow range of (2-7) x 10(- 3). The other one-third show trends of decreasing L-[CII]/L-FIR with i ncreasing dust temperature, as measured by the flux ratio of infrared emission at 60 and 100 mu m, F-v(60 mu m)/F-v(100 mu m), and with incr easing star formation activity, measured by the ratio of FIR and blue- band luminosity, L-FIR/L-B. We also find three FIR-bright galaxies tha t are deficient in the [C II] line, which is undetected with 3 sigma u pper limits of L-[CII]/L-FIR < (0.5-2) x 10(-4). The trend in the L-[C II]/L-FIR ratio with the temperature of dust and with star formation a ctivity may be due to decreased efficiency of photoelectric heating of gas at high UV radiation intensity as dust grains become positively c harged, decreasing the yield and the energy of the photoelectrons. The three galaxies with no observed photodissociation region lines have a mong the highest L-FIR/L-B and F-v(60 mu m)/F-v(100 mu m) ratios. Thei r lack of [C II] lines may be due to a continuing trend of decreasing L-[CII]/L-FIR with increasing star formation activity and dust tempera ture seen in one-third of the sample with warm IRAS colors. In that ca se, the upper limits on L-[CII]/L-FIR imply a ratio of UV flux to gas density of G(0)/(n) > 10 cm(3) (where G(0) is in units of the local av erage interstellar field). The low L-[CII]/L-FIR ratio could also be d ue to either weak [CII], owing to self-absorption, or a strong FIR con tinuum from regions weak in [C II], such as dense H II regions or plas ma ionized by hard radiation of active galactic nuclei. The mid-infrar ed and radio images of these galaxies show that most of the emission c omes from a compact nucleus. CO and H I are detected in these galaxies , with H I seen in absorption toward the nucleus.