Darek C. Lis 1, Y. Li 1, Karl M. Menten 2, & C.D. Dowell 1
1 Caltech, Pasadena, USA
2 Max-Planck-Institut für Radioastronomie, Bonn, Germany
We have used the Long Wavelength Spectrometer aboard of ISO in the
grating mode to map the FIR continuum emission (45-175 
)
toward a
string of massive GMC cores near the Galactic center, which have been
detected via their submillimeter continuum emission with the Caltech
Submillimeter Observatory. These cores are observed in emission at FIR
wavelengths (
). However, at MIR wavelengths (
)
they
are seen in absorption against the general Galactic center background.
This indicates that they are located in front of the bulk of the dust
responsible for the diffuse FIR emission, most likely a few hundred pc
from the Galactic center. Gray-body 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 the
most massive core, GCM0.25+0.11, has a mean temperature of about 26 K
and a 100 mic optical depth of about 0.17. By subtracting the spectrum
of the diffuse component from the spectrum observed toward
GCM0.25+0.11, we have been able to extract the intrinsic dust spectrum
of this GMC core. Gray-body fits to the resulting far-infrared
spectrum combined with the submillimeter measurements (350-800 )
carried out with the CSO give a low temperature (about 18 K) for the
bulk of the dust, much lower than that in the well studied Galactic
center continuum sources like Sgr B2, C, or D. Some of the other
sources we have studied with ISO are even colder (13-19 K average dust
temperature). In addition, the grain emissivity in these GMC cores is
a very steep function of frequency with the power of 2.5-3.5,
indicating the presence of dust grains covered with thick ice mantles.
The GMC cores we have studied with ISO appear distinctly different from the well known Galactic center dust continuum sources, like Sgr C, Sgr D, and the `20 km/s' and `50 km/s' clouds in Sgr A. All these sources have comparable continuum fluxes, and thus H2 column densities and masses, to those of GCM0.25+0.11. Yet they all harbor sites of high-mass star formation, as evidenced by the presence of of ultracompact HII regions or embedded FIR sources. We have found no evidence for ongoing high-mass star formation associated with GCM0.25+0.11, in spite of its high mass and gas density. However, since the well studied Galactic center clouds were all selected based on their strong FIR or radio emission, it remains unclear how representative they are for the General Galactic center GMC population.
In addition to the FIR continuum emission, we have mapped with ISO the distribution of the atomic fine structure lines of CII and OI in the vicinity of the dust cores. The observed CII/OI and (CII+OI)/IR flux ratios are consistent with a mean hydrogen density of 600-1000 atoms per cubic centimeter and the incident UV flux of 600-1300 times the local Galactic value.