CH 3 GHz Observations of Molecular Clouds along the Galactic Plane

Abstract

Spectra in the CH 2 Pi 1/2, J=1/2, F=1-1 transition at 3335 MHz were obtained in three five-point crosses centered on the Galactic plane at l=50 deg, 100∞, and 110∞. The lines of sight traversed both giant molecular clouds (GMCs) and local, smaller entities. This transition is a good tracer of low-density molecular gas, and the line profiles are very similar to CO(1-0) data at nearly the same resolution. In addition, the CH 3335 MHz line can be used to calibrate the CO-H 2 conversion factor (X CO) in low-density molecular gas. Although this technique underestimates X CO in GMCs, our results are within a factor of 2 of X CO values calibrated for GMCs by other techniques. The similarity of CH and CO line profiles, and that of X CO values derived from CH and more traditional techniques, implies that most of the molecular gas along the observed lines of sight is at relatively low densities (n 3 cm -3).