Infrared Space Observatory Measurements of [C II] Line Variations in Galaxies

December 1997 • 1997ApJ...491L..27M

Authors • Malhotra, S. • Helou, G. • Stacey, G. • Hollenbach, D. • Lord, S. • Beichman, C. A. • Dinerstein, H. • Hunter, D. A. • Lo, K. Y. • Lu, N. Y. • Rubin, R. H. • Silbermann, N. • Thronson, H. A., Jr. • Werner, M. W.

Abstract • 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, L_[CII]/L_FIR, 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 L_[CII]/L_FIR ratios in the narrow range of (2-7)×10^-3. The other one-third show trends of decreasing L_[CII]/L_FIR with increasing dust temperature, as measured by the flux ratio of infrared emission at 60 and 100 μm, F_ν(60 μm)/F_ν(100 μm), and with increasing star formation activity, measured by the ratio of FIR and blue-band luminosity, L_FIR/L_B. We also find three FIR-bright galaxies that are deficient in the [C II] line, which is undetected with 3 σ upper limits of L_[CII]/L_FIR<(0.5-2)×10^-4.

The trend in the L_[CII]/L_FIR ratio with the temperature of dust and with star formation activity may be due to decreased efficiency of photoelectric heating of gas at high UV radiation intensity as dust grains become positively charged, decreasing the yield and the energy of the photoelectrons. The three galaxies with no observed photodissociation region lines have among the highest L_FIR/L_B and F_ν(60 μm)/F_ν(100 μm) ratios. Their 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 temperature seen in one-third of the sample with warm IRAS colors. In that case, the upper limits on L_[CII]/L_FIR imply a ratio of UV flux to gas density of G₀/n>10 cm³ (where G₀ is in units of the local average interstellar field). The low L_[CII]/L_FIR ratio could also be due to either weak [C II], owing to self-absorption, or a strong FIR continuum from regions weak in [C II], such as dense H II regions or plasma ionized by hard radiation of active galactic nuclei. The mid-infrared and radio images of these galaxies show that most of the emission comes from a compact nucleus. CO and H I are detected in these galaxies, with H I seen in absorption toward the nucleus.

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George Helou

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