Far-Infrared Observation of Metal-Poor Interstellar Medium

The purpose of this research is to diagnose interstellar gas in nearby galaxies and to understand the ionization/dissociation structure of the interstellar space of the Galaxy as well as the energy transfer flow there by line spectroscopy in the far-infrared region. At first, we have developed a new type reflection grating, 'immersion grating1, which is relatively small although it has higher spectral resolution. By using silicon whose refractive index is 3.4, the spectral resolving power of 10,000 is achieved with a 10 cm grating. The anti-reflection coating is also technically feasible. We have measured the transmission of a large silicon material that is to be actually used for the grating at a cryogenic temperature (4K) by developing a new device that is necessary for this kind of measurement, and the result is more than 50 %. Moreover, we designed a two-dimensional array as well as a cryogenic optical system for the spectrometer. In parallel to this, by analyzing the data taken by the ISO (ESA) satellite, we have an important knowledge about the silicon abundance in the interstellar space of the Carina region, a typical large-scale star-forming region. We also found a new method to estimate the physical characteristics of the interstellar dust grain from a fewer observational data in the far-infrared region. These result were submitted to main journals.