Quiescent giant molecular cloud cores in the Galactic center

We have used the Long Wavelength Spectrometer aboard the Infrared Space Observatory to map the far-infrared continuum emission (45-175 mum) toward several massive giant molecular cloud (GMC) cores located near the Galactic center. The observed far-infrared and submillimeter spectral energy distributions imply low temperatures (similar to 15-22 K) for the bulk of the dust in all the sources, which is consistent with external heating by the diffuse interstellar radiation field (ISRF) and suggests that these GMCs do not harbor high-mass star formation sites, despite their large molecular mass. Observations of far-infrared atomic fine-structure lines of C II and O I indicate an ISRF enhancement of similar to 10(3) in the region. Through continuum radiative transfer modeling, we show that this radiation field strength is in agreement with the observed far-infrared and submillimeter spectral energy distributions, assuming primarily external heating of the dust with only limited internal luminosity (similar to2 x 10(5) L.). Spectroscopic observations of millimeter-wave transitions of H2CO, CS, and (CS)-S-34 carried out with the Caltech Sub-millimeter Observatory and the Institut de Radio Astronomie Millimetrique (IRAM) 30 m telescope indicate a gas temperature of similar to 80 K, which is significantly higher than the dust temperature, and a density of similar to1 x 10(5) cm(-3) in GCM 0.25+0.01, the brightest submillimeter source in the region. We suggest that shocks caused by cloud collisions in the turbulent interstellar medium in the Galactic center region are responsible for heating the molecular gas. This conclusion is supported by the presence of widespread emission from molecules such as SiO, SO, and CH3OH, which are considered good shock tracers. We also suggest that the GMCs studied here are representative of the "typical," pre-star-forming cloud population in the Galactic center.