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 5-point crosses centered on the Galactic plane at $\\ell =$ 50$\\arcdeg$, 100$\\arcdeg$, and 110$\\arcdeg$. The lines of sight traverse 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$_{\\rm CO}$) in low-density molecular gas. Although this technique underestimates X$_{\\rm CO}$ in GMCs, our results are within a factor of two of X$_{\\rm CO}$ values calibrated for GMCs by other techniques. The similarity of CH and CO line profiles, and that of X$_{\\rm CO}$ 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 ($n \\le$ 10$^3$ cm$^{-3}$).