OH Survey along Sightlines of Galactic Observations of Terahertz C+

Abstract

We have obtained OH spectra of four transitions in the ${}^2{\Pi }_{3/2}$ ground state, at 1612, 1665, 1667, and 1720 MHz, toward 51 sightlines that were observed in the Herschel project Galactic Observations of Terahertz C+. The observations cover the longitude range of (32${}^{\circ }$, 64${}^{\circ }$) and (189${}^{\circ }$, 207${}^{\circ }$) in the northern Galactic plane. All of the diffuse OH emissions conform to the so-called 'Sum Rule' of the four brightness temperatures, indicating optically thin emission condition for OH from diffuse clouds in the Galactic plane. The column densities of the HI `halos' N(HI) surrounding molecular clouds increase monotonically with OH column density, N(OH), until saturating when N(HI)=1.0 x 10${}^{21}$ cm${}^{-2}$ and N (OH) $\ge 4.5\times 10^{15}$ cm${}^{-2}$, indicating the presence of molecular gas that cannot be traced by HI. Such a linear correlation, albeit weak, is suggestive of HI halos' contribution to the UV shielding required for molecular formation. About 18% of OH clouds have no associated CO emission (CO-dark) at a sensitivity of 0.07 K but are associated with C${}^{+}$ emission. A weak correlation exists between C${}^{+}$ intensity and OH column density for CO-dark molecular clouds. These results imply that OH seems to be a better tracer of molecular gas than CO in diffuse molecular regions.

arXiv comment13 pages, 12 figures, accepted for publication in ApJ

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