Imprints of Molecular Clouds in Radio Continuum Images

TL;DR

This paper introduces 'radio dark clouds', a new method using radio continuum images to identify dense molecular clouds embedded in UV or cosmic ray environments, providing insights into their depth and magnetic fields.

Contribution

The study presents a novel technique for detecting cold molecular clouds via radio continuum deficits, enhancing understanding of their physical properties and interactions in the galaxy.

Findings

Radio dark clouds correlate with dense molecular regions.

The depth of clouds is comparable to the continuum emission size.

High-resolution radio images effectively probe molecular cloud interactions.

Abstract

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.