Global physical conditions of the interstellar medium in nearby galaxies

Far-infrared spectra (43-197 mum) of 34 nearby galaxies obtained by the Lon g 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 [C II]158 mum, [O I]63 mum, [N II]122 mum, and [O III]88 mum, wer e detected in most of the sample galaxies. [O I]145 mum line was detected i n 13 galaxies. The line fluxes of [C II]158 mum and[N II]122 mum relative t o the total far-infrared flux (FIR) decrease as the far-infrared color beco mes bluer, while the ratio of the [O I]63 mum flux to FIR does not show a s ystematic trend with the color. The [O III]88 mum to FIR ratio shows a larg e scatter with a weak trend of increase with the color. AGNs do not show an y distinguishable trend from normal and starburst galaxies in the far-infra red 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 fa r-ultraviolet radiation field intensity G(0) and the gas density n by assum ing that all the observed [O I]63 mum and far-infrared continuum emissions come from PDRs. Comparison with PDR models indicates that G(0) ranges from 10(2)-10(4) and n similar to 10(2)-10(4) cm(-3). The present results also s uggest that n varies proportionally with G(0). The ratio of [C II] 158 mum to CO (J = 1-0) line emission supports the linear increase in n with G(0). We estimate that about a half of [C II]158 mum emission originates from PDR s and attribute the rest to the emission as coming from low-density diffuse ionized gas. The estimated intensity of [C II]158 mum from the ionized gas is compatible with the observed intensity of [N II]122 mum if both lines c ome from the same diffuse ionized gas. The present analysis suggests that t he decrease in [C II]158 mum/FIR with the far-infrared color may not be acc ounted for by the decrease in the photoelectric heating efficiency owing to the increase in positive charges of dust grains because a measure of the e fficiency, G(0)/n, is found to stay constant with the far-infrared color. I nstead the decrease can be interpreted in terms of either the increase in t he collisional de-excitation of the [C ii] transition due to the increase i n the gas density or the decrease in the ionized component relative to the far-infrared intensity suggested by the decrease in [N II]122 mum/FIR. Base d on the present analysis, we derive average relations of the far-infrared color with G(0) and n in galaxies, which can be applied to the investigatio n of interstellar matter in distant galaxies.