Imprints of Molecular Clouds in Radio Continuum Images
  We show radio continuum images of several molecular complexes in the inner
Galaxy and report the presence of dark features that coincide with dense
molecular clouds. Unlike infrared dark clouds, these features which we call
"radio dark clouds" are produced by a deficiency in radio continuum emission
from molecular clouds that are embedded in a bath of UV radiation field or
synchrotron emitting cosmic ray particles. The contribution of the continuum
emission along different pathlengths results in dark features that trace
embedded molecular clouds. The new technique of identifying cold clouds can
place constraints on the depth and the magnetic field of molecular clouds when
compared to those of the surrounding hot plasma radiating at radio wavelengths.
The study of five molecular complexes in the inner Galaxy, Sgr A, Sgr B2, radio
Arc, the snake filament and G359.75-0.13 demonstrate an anti--correlation
between the distributions of radio continuum and molecular line and dust
emission. Radio dark clouds are identified in GBT maps and VLA images taken
with uniform sampling of {\it uv} coverage. The level at which the continuum
flux is suppressed in these sources suggests that the depth of the molecular
cloud is similar to the size of the continuum emission within a factor of two.
These examples suggest that high resolution, high dynamic range continuum
images can be powerful probes of interacting molecular clouds with massive
stars and supernova remnants in regions where the kinematic distance estimates
are ambiguous as well as in the nuclei of active galaxies.
