Far-infrared spectroscopic observations of the Galactic center region^*

¹ Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510, Japan e-mail: yasuda@ir.isas.jaxa.jp
² The Graduate University for Advanced Studies, Shonan Village, Hayama, Kanagawa 240-193, Japan
³ Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands

Received: 28 August 2007
Accepted: 11 December 2007

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.