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 km) toward several massive giant molecular cloud (GMC) cores located near the Galactic center. The observed far-infrared and submillimeter spectral energy distribu- tions imply low temperatures (D15¨22 K) for the bulk of the dust in all the sources, which is consistent with external heating by the diÜuse interstellar radiation —eld (ISRF) and suggests that these GMCs do not harbor high-mass star formation sites, despite their large molecular mass. Observations of far- infrared atomic —ne-structure lines of C II and O I indicate an ISRF enhancement of D103 in the region. Through continuum radiative transfer modeling, we show that this radiation —eld strength is in agree- ment with the observed far-infrared and submillimeter spectral energy distributions, assuming primarily external heating of the dust with only limited internal luminosity (D2 ) 105 Spectroscopic obser- L _ ). vations of millimeter-wave transitions of CS, and C34S carried out with the Caltech Sub- H 2 CO, millimeter Observatory and the Institut de Radio Astronomie (IRAM) 30 m telescope Millimeç trique indicate a gas temperature of D80 K, which is signi—cantly higher than the dust temperature, and a density of D1 ) 105 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 which are considered good shock CH 3 OH, tracers. We also suggest that the GMCs studied here are representative of the ìì typical,ˇˇ prestar-forming cloud population in the Galactic center. Subject headings: Galaxy: centerinfrared: ISMISM: clouds ¨ ISM: individual (GCM 0.25)0.11, Sagittarius B1) ¨ stars: formation