CH 3 GHz observations of molecular clouds along the Galactic plane

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 degrees, 100 degrees, and 110 degrees. 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 <= 10(3) cm(-3)).