Multi-Line Survey Observation of the Galactic Plane by Balloon-Borne Telescopes

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It has widely been demonstrated that there exist various stages of gas, such as atoms, ions, and molecules in the interstellar space of the Galaxy. Among them, hydrogen 21 cm line and CO 2.6 mm line have been playing important roles in investigations of the galactic structure and physical and chemical stage of the Galaxy through the neutral hydrogen atomic gas and molecular gas, respectively. However, the previous result of our study by a [CII] far-Infrared line survey showed that there exists a new interstellar gas phase, "diffuse photodissociation region" . Photodissociation regions in the enterface region between the CO molecular gas and the hydrogen atomic gas, and are the dominant coolant by conversion of UV and optical radiation from luminous stars into the far-infrared radiation.Therefore, the far-Infrared spectral lines are key for understanding the galactic interstellar space/matter, and are a tool for measuring the interstellar heavy element abundance. We have carried out a wide sky survey of [CII] 158 micron by a space infrared telescope ( IRTS ) and by a balloon-borne telescope ( BICE ), and [OI] 63 micron, [OIII] 52,88 micron, [CII] 158 micron, [NII] 122 micron, and [NIII] 57 micron lines by a European Infrared space observatory ( ISO ). From these data, we have shown diffuse photodissociation regions and diffuse ionization regions are important gas phases in the Galactic Interstellar space.