Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997apj...491l..27m&link_type=abstract
Astrophysical Journal Letters v.491, p.L27
Astronomy and Astrophysics
Astronomy
94
Radiation Mechanisms: Thermal, Ism: Atoms, Galaxies: Ism, Infrared: Ism: Lines And Bands
Scientific paper
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 Spectrometer (LWS) on the Infrared Space Observatory (ISO). The ratio of the line to total far-infrared (FIR) luminosity, L_{[{C} {II}]}/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_{[{C} {II}]}/L_{{FIR}} ratios in the narrow range of (2-7)x10^{-3} . The other one-third show trends of decreasing L_{[{C} {II}]}/L_{{FIR}} with increasing dust temperature, as measured by the flux ratio of infrared emission at 60 and 100 mu m, F_ν(60 mu {m})/F_ν(100 mu {m}) , and with increasing star formation activity, measured by the ratio of FIR and blue-band luminosity, L_{{{FIR}}}/LB . We also find three FIR-bright galaxies that are deficient in the [C II] line, which is undetected with 3 sigma upper limits of L_{[{C} {II}]}/L_{{FIR}}<(0.5-2)x10^{-4} . The trend in the L_{[{C} {II}]}/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}}}/LB and F_ν(60 mu {m})/F_ν(100 mu {m}) ratios. Their lack of [C II] lines may be due to a continuing trend of decreasing L_{[{C} {II}]}/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_{[{C} {II}]}/L_{{FIR}} imply a ratio of UV flux to gas density of G0/n>10 cm 3 (where G0 is in units of the local average interstellar field). The low L_{[{C} {II}]}/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.
Beichman Charles Arnold
Dinerstein Harriet
Helou George
Hollenbach David
Hunter Deidre Ann
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