Global physical conditions of the interstellar medium in nearby galaxies

Far-infrared spectra (43{197 m) 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 in- terstellar matter in galaxies. The present sample includes not only normal galaxies but also starbursts and ac- tive galactic nuclei (AGNs). Far-infrared forbidden lines, such as (C ii)158 m, (O i)63 m, (N ii)122 m, and (O iii)88 m, were detected in most of the sample galaxies. (O i)145 m line was detected in 13 galaxies. The line fluxes of (C ii)158 ma nd (Nii)122 m relative to the total far-infrared flux (FIR) decrease as the far-infrared color becomes bluer, while the ratio of the (O i)63 m flux to FIR does not show a systematic trend with the color. The (O iii)88 mt oFIR 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 eld intensity G0 and the gas density n by assuming that all the observed (O i)63 m and far-infrared continuum emissions come from PDRs. Comparison with PDR models indicates that G0 ranges from 10 2 {10 4 and n 10 2 { 10 4 cm 3 . The present results also suggest that n varies proportionally with G0. The ratio of (C ii) 158 mt o CO (J = 1{0) line emission supports the linear increase in n with G0. We estimate that about a half of (C ii)158 m emission originates from PDRs and attribute the rest to the emission as coming from low-density diuse ionized gas. The estimated intensity of (C ii)158 m from the ionized gas is compatible with the observed intensity of (N ii)122 m if both lines come from the same diuse ionized gas. The present analysis suggests that the decrease in (C ii)158 m=FIR with the far-infrared color may not be accounted for by the decrease in the photoelectric heating eciency owing to the increase in positive charges of dust grains because a measure of the eciency, 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 (C ii) transition due to the increase in the gas den- sity or the decrease in the ionized component relative to the far-infrared intensity suggested by the decrease in (N ii)122 m=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.