2001A&A...375..566N

Far-infrared spectra (43-197µm) of 34 nearby galaxies obtained by the Long Wavelength Spectrometer (LWS) aboard the Infrared Space Observatory (ISO) were analyzed to investigate the general properties of interstellar matter in galaxies. The present sample includes not only normal galaxies but also starbursts and active galactic nuclei (AGNs). Far-infrared forbidden lines, such as [CII]158µm, [OI]63µm, [NII]122µm, and [OIII]88µm, were detected in most of the sample galaxies. [OI]145µm line was detected in 13 galaxies. The line fluxes of [CII]158µm and [NII]122µm relative to the total far-infrared flux (FIR) decrease as the far-infrared color becomes bluer, while the ratio of the [OI]63µm flux to FIR does not show a systematic trend with the color. The [OIII]88µm to FIR ratio shows a large scatter with a weak trend of increase with the color. AGNs do not show any distinguishable trend from normal and starburst galaxies in the far-infrared spectra, suggesting that the far-infrared emission is mainly driven by star-formation activities even in AGNs. We estimate the physical conditions of photodissociation regions (PDRs) in the sample galaxies, such as the far-ultraviolet radiation field intensity G₀ and the gas density n by assuming that all the observed [OI]63µm and far-infrared continuum emissions come from PDRs. Comparison with PDR models indicates that G₀ ranges from 10²-10⁴ and n∼10²-10⁴cm^–3. The present results also suggest that n varies proportionally with G₀. The ratio of [CII]158µm to CO (J=1-0) line emission supports the linear increase in n with G₀. We estimate that about a half of [CII]158µm emission originates from PDRs and attribute the rest to the emission as coming from low-density diffuse ionized gas. The estimated intensity of [CII]158µm from the ionized gas is compatible with the observed intensity of [NII]122µm if both lines come from the same diffuse ionized gas. The present analysis suggests that the decrease in [CII]158µm/FIR with the far-infrared color may not be accounted for by the decrease in the photoelectric heating efficiency owing to the increase in positive charges of dust grains because a measure of the efficiency, G₀/n, is found to stay constant with the far-infrared color. Instead the decrease can be interpreted in terms of either the increase in the collisional de-excitation of the [CII] transition due to the increase in the gas density or the decrease in the ionized component relative to the far-infrared intensity suggested by the decrease in [NII]122µm/FIR. Based on the present analysis, we derive average relations of the far-infrared color with G₀ and n in galaxies, which can be applied to the investigation of interstellar matter in distant galaxies.