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 wereobtained in three 5-point crosses centered on the Galactic plane at $\ell =$50$\arcdeg$, 100$\arcdeg$, and 110$\arcdeg$. The lines of sight traverse bothGiant Molecular Clouds (GMCs) and local, smaller entities. This transition is agood tracer of low-density molecular gas and the line profiles are very similarto CO(1-0) data at nearly the same resolution. In addition, the CH 3335 MHzline can be used to calibrate the CO-H$_2$ conversion factor (X$_{\rm CO}$) inlow-density molecular gas. Although this technique underestimates X$_{\rm CO}$in GMCs, our results are within a factor of two of X$_{\rm CO}$ valuescalibrated for GMCs by other techniques. The similarity of CH and CO lineprofiles, and that of X$_{\rm CO}$ values derived from CH and more traditionaltechniques, implies that most of the molecular gas along the observed lines ofsight is at relatively low densities ($n \le$ 10$^3$ cm$^{-3}$).Comment: 26 pages, 12 figures, submitted to the AJ, revised after referee repor