Far-infrared spectroscopic observations of the Galactic center region

Abstract

Context: The Galactic center region is luminous in the far-infrared (FIR), but dominant sources of this luminosity are still controversial.
Aims: We investigate physical conditions of the diffuse interstellar medium in the Galactic center region by multi-line spectroscopy in the FIR.
Methods: 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 H II regions, including [O I] and [C II] lines that are dominant coolants of interstellar clouds.
Results: The observations show that the [C II]/FIR ratio is systematically low whereas the [O I]/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 ± 300 K in the Galactic center region, which is significantly lower than 8600 ± 500 K in the Galactic disk region.
Conclusions: 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.

This work is based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA.