2008A&A...480..157Y

The Galactic center region is luminous in the far-infrared (FIR), but dominant sources of this luminosity are still controversial. We investigate physical conditions of the diffuse interstellar medium in the Galactic center region by multi-line spectroscopy in the FIR. We analyze the archival data of the Galactic center region obtained with the Long-Wavelength Spectrometer on board the Infrared Space Observatory. We detect strong continuum emission and fine-structure emission lines from photo-dissociation and HII regions, including [OI] and [CII] lines that are dominant coolants of interstellar clouds. The observations show that the [CII]/FIR ratio is systematically low whereas the [OI]/FIR ratio is almost constant toward the Galactic center region. By using a photo-dissociation region model with observed FIR parameters, we obtain radiation an effective temperature of 6600±300K in the Galactic center region, which is significantly lower than 8600±500K in the Galactic disk region. Dominant sources of the FIR luminosity in the Galactic center region are not likely to be young OB stars but rather cool stars, K and M giants, which implies that the current star-formation activity is rather low in the Galactic center region.