Infrared Space Observatory Long Wavelength Spectrometer Observations of a Cold Giant Molecular Cloud Core near the Galactic Center

Abstract

We have used the Long Wavelength Spectrometer aboard the Infrared Space Observatory in the grating mode to map the far-infrared continuum emission (45-175 μm) toward the massive giant molecular cloud core GCM 0.25+0.11 located near the Galactic center. Graybody models of the observed far-infrared spectral energy distribution indicate that the bulk of the dust in the diffuse component along the line of sight toward GCM 0.25+0.11 has a mean temperature of ~26 K and a 100 μm optical depth of ~0.17. GCM 0.25+0.11 is observed in emission at far-infrared (FIR) wavelengths (100 μm). However at midinfrared wavelengths (70 μm) the core is seen in absorption against the general Galactic center background. This indicates that GCM 0.25+0.11 is located in front of the bulk of the dust responsible for the diffuse FIR emission, most likely a few hundred parsecs from the Galactic center. By subtracting the spectrum of the diffuse component from the spectrum observed toward GCM 0.25+0.11, we have been able to extract the intrinsic spectrum of this GMC core. Graybody fits to the resulting far-infrared spectrum combined with our previous submillimeter measurements (350-800 μm) give a low temperature ~18 K for the bulk of the dust in the GCM 0.25+0.11 core. In addition, the grain emissivity is a very steep function of frequency (ν2.8). The high grain emissivity exponent is consistent with the presence of dust grains covered with thick ice mantles. We have complemented our ISO data with CO (2-1) and HCO+ (3-2) observations carried out with the Caltech Submillimeter Observatory. The molecular emission shows a large velocity gradient across the southern part of the core indicative of streaming motions of the gas or of the presence of multiple, spatially overlapping velocity components. The observed gas kinematics may indicate that GCM 0.25+0.11 is in the process of being disrupted by the strong tidal forces caused by the high mass concentration in the Galactic center region. This might explain why there is no evidence for ongoing high-mass star formation associated with this core, in spite of its large molecular mass. However, the mean H2 density of GCM 0.25+0.11 is well above the tidal stability limit for a Galactocentric distance of a few hundred parsecs implied by our observations. An alternative explanation is that we are witnessing the very early stage of a cloud-cloud collision that may result in a future star formation episode.