Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000isass..14...91n&link_type=abstract
The Institute of Space and Astronautical Science Report SP No. 14, p. 91-98.
Astronomy and Astrophysics
Astronomy
Galaxies, Far-Infrared Spectra, Ism Global Properties
Scientific paper
Far-infrared spectra (45-189 micron) of 36 nearby galaxies taken by the Long Wavelength Spectrometer (LWS) aboard the Infrared Space Observatory (ISO) were analyzed to investigate the general properties of interstellar matter in galaxies. Far-infrared forbidden lines, such as [CII] 158 micron, [OI] 63 micron, and [NII] 122 micron, were detected in most of the sample galaxies. The line intensities of [CII] 158 micron and [NII] 122 micron relative to the far-infrared intensity (FIR) decrease as the far-infrared color becomes bluer, while the ratio of the [OI] intensity to FIR does not show a systematic trend with the color. We estimate the contribution from photodissociation regions (PDRs) to the observed [CII] 158 micron intensity, assuming that all the [OI] 63 micron and far-infrared continuum emission comes from the PDRs. The results indicate that the fraction of [CII] 158 micron emission arising from the PDRs ranges from 30% to 80% among the sample galaxies. The rest of the emission is suggested to originate from the extended low-density warm ionized medium (ELDWIM). The derived neutral hydrogen gas density increases as the far-infrared color becomes bluer, being compatible with the idea that active galaxies have dense gas in their interstellar medium. The contribution from ELDWIM to [CII] 158 micron/FIR does not show any systematic trend with the far-infrared color. The present results suggest that the relation between [CII] 158 micron/FIR and the far-infrared color can be attributed to the decrease in [CII] emission from the PDRs owing to the increase in collisional de-excitation rather than the decrease in the photoelectric heating efficiency.
Negishi Taketoshi
Onaka Takashi
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