Global physical conditions of the interstellar medium in nearby galaxies

Abstract

Far-infrared spectra (43-197 mu 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 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 mu m, [O I]63 mu m, [N Ii]122 mu m, and [O Iii]88 mu m, were detected in most of the sample galaxies. [O I]145 mu m line was detected in 13 galaxies. The line fluxes of [C Ii]158 mu m and [N Ii]122 mu 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 mu m flux to FIR does not show a systematic trend with the color. The [O Iii]88 mu m 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 [O I]63 mu m and far-infrared continuum emissions come from PDRs. Comparison with PDR models indicates that G₀ ranges from 10²-10⁴ and n ~ 10²-10⁴ cm^-3. The present results also suggest that n varies proportionally with G₀. The ratio of [C Ii] 158 mu m to CO (J=1-0) line emission supports the linear increase in n with G₀. We estimate that about a half of [C Ii]158 mu m emission originates from PDRs and attribute the rest to the emission as coming from low-density diffuse ionized gas. The estimated intensity of [C Ii]158 mu m from the ionized gas is compatible with the observed intensity of [N Ii]122 mu m if both lines come from the same diffuse ionized gas. The present analysis suggests that the decrease in [C Ii]158 mu m/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 [C Ii] 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 [N Ii]122 mu 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. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, The Netherlands and the UK) and with the participation of ISAS and NASA.