@misc{oai:ir.soken.ac.jp:00000554, author = {安田, 晃子 and ヤスダ, アキコ and YASUDA, Akiko}, month = {2016-02-17}, note = {The Galactic center region (～500 pc) is luminous in the far-infrared(FIR),however dominantF Heating sources of its luminosity are still controversial. There are many pieces of evidence for active star-formation in some areas in the Galactic center region such as the Sgr B2 HII) region. On the other hand, it is proposed that dominant heating sources for the dust in the general Galactic Center region may not be young OB stars but the population of cool stars, K and M giants (Cox & Laureijs 1989 ). This is supported by the large infrared excess (IRE), which is defined as infrared luminosity over Lyman alpha luminosity and is an indicator of the star-formation activity. The IRE is ～30 in the Galactic center region, while it is ～10 in typical HII region in the Galatic disk. This suggests that the star-formation in the Galactic center region is currently inactive as a whole.
　　Previous Galactic ［CII］line observations revealed that the ［CII］emission did not show a domi-nant peak in the Galactic center (e.g. Bennett et al.1994; Nakagawa et al.1995,1998). Especially,observations using the Balloon-borne Infrared Carbon Explore (BICE) showed that the ratio of the［CII］line to the FIR continuum emission was systematically low toward the Galactic center. Naka-gawa et al. (1995, 1998) suggested that the low ［CII］/FIR ratio was caused by softer UV radiation in the Galactic center region than in the Galactic disk, supporting that the luminosity of the Galactic center region may be attributed to late-type stars. However, because the observations were made in the ［CII］line alone, they could not rule out other possibilities to explain the low ［CII］/FIR ratio in the Galactic center and multi-line spectroscopy was required to resolve their problem conclusively.
　　In this thesis, in order to reveal the dominant heating source of the FIR luminosity, the physical condition of the ISM in both general and local regions of the Galactic center has been investigated in detail by multi-line spectroscopy in the FIR. The FIR spectral region is rich with many spectral lines that are important for the study of interstellar physics and chemistry. The spectroscopic date are obtained for large-scale areas along the Galactic plane with the LWS on ISO as well as small-scale mapping observations of Galactic center clouds by the FIS/FTS on AKARI and the LWS;the ISO date were taken from the archives, while the AKARI date were derived from observations for calibration of the FIS/FTS.
　　The observation consist of two types: one is large-scale observations and the other is small-scale observations. For the large-scale observations, the date were selected for the area covering |ｌ|< 10゜and |b|< 0.1゜and least contaminated by strong HII regions, i.e., on-plane general diffuse regions.The results show that the ratio of the ［CII］to the FIR continuum emission is systematically low toward the Galactic center, whereas that of the ［OI］to the FIR is almost constant; the former result is consistent with that of the previous BICE-line mapping(Nagasawa et al. 1995, 1998). The physical parameters of the ISM in the Galactic center region have been obtained by using the PDR model (Spaans et al.1994) that takes account of variation of an effective temperature of interstellar radiation field. The results show that the radiation effective temperatures are significantly low in the Galactic center region; the radiation effective temperature of 6600 ± 300 K in the Galactic center region and 8600 ± 500 K in the Galactic disk region, while similar hydrogen densities ofⁿH～10^2.8-3.0 cm ^-3 are obtained for both regions. It is therefore concluded that the suppression of the ［CII］/FIR ratio toward the Galactic center is most probably due to soft radiation field in the Galactic center region. The present study has unambiguously identified the cause of the low［CII］/FIR ratio by the multiple-line spectroscopy.
　　To discuss the physical implications of the soft radiation field in the Galactic center region, two star-formation models are introduced, where the numbers of H- and C-ionizing photons produced by stars are calculated and effective color temperatures of the radiation field from ratios of these photons are derived. As a result, the observed effective temperature supports the model in which star-formation activities in the Galactic center region started a long time ago (1×10⁹yr )and turned off in the past. Hence it is concluded that dominant sources of the FIR luminosity are not likely to be young OB stars but rather cool stars, K and M giants, which implies that current star-formation activity is rather low in the Galactic center region.
　　As for the small-scale observations, the ISO date were selected from the mapping observations of the area including the Sgr B1 region, the giant molecular clouds in the Ridge region, and the Pistol region. The AKARI mapping date cover the Pistol region and the Arches cluster region; on the basis of the Pistol region overlapped with ISO, the absolute line fluxes of FIS/FTS date have been calibrated. The Sgr B1, the Pistol, and the Arches cluster regions are representative of star-forming regions, while the giant molecular clouds in the Ridge are representative of quiescent regions.　Throughout these small-scale observation, the ［CII］line intensity shows good liner correlation with the ［OI］ line intensity, suggesting that the［CII］line originates mostly from PDRs not form HII regions. It is also found that local variations in both ［CII］/FIR and OI/FIR ratios are very small as compared to the ［OIII］ / FIR ratio, which ensures that ［CII］and ［OI］ lines are not concentrated in HII region and the results of the large-scale observations are not affected by local variations. From the Ridge region, the results show lower G0/ but even smaller ［CII］/FIR ratios than those in the large-scale Galactic center region, while the ［OI］/FIR in the Ridge region are similar to those in the large-scale region. Hence it is suggested that the radiation field in the Ridge region is consistent with, or even softer than that obtained from the large-scale observations.
　　Finally, from the AKARI mapping data of the Arches cluster regions, the detailed structures of the ISM are discussed. The marked differences in the spatial distribution among the［OIII］,［NII］,and ［CII］ emission lines are found in the area near the Arches cluster; the ［OIII］ distribution has a peak near the cluster and spatially corresponds well to one of the radio filaments seen in this region. The ［NII］ distribution has peak at the position different from that of the ［OIII］distribution, located father away from the Arches cluster, and the ［CII］ distribution is extended farthest among them. This result shows that the emission of the emission of the ionic line with higher ionization potential is distributed closer to the Arches cluster, which is the first direct evidence that UV photons from Arches cluster are ionizing the surface of the nearby molecular cloud forming the radio filament and penetrating deeper to dissociate the cloud., 総研大甲第1132号}, title = {Far-Infrared Spectroscopic Observations of the Galactic Center with ISO and AKARI}, year = {} }