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${}_{\mathrm{CO}}$) in low-density molecular gas. Although this technique underestimates X${}_{\mathrm{CO}}$ in GMCs, our results are within a factor of two of X${}_{\mathrm{CO}}$ values calibrated for GMCs by other techniques. The similarity of CH and CO line profiles, and that of X${}_{\mathrm{CO}}$ 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${}^{-3}$).

26 pages, 12 figures, submitted to the AJ, revised after referee report