Abstract-OH Survey along Sightlines of Galactic Observations of Terahertz C+

Ningyu Tang, Di Li, Carl Heiles, Nannan Yue, J. R. Dawson, Paul F. Goldsmith, Marko Krčo, N. M. McClure-Griffiths, Shen Wang, Pei Zuo, Jorge L. Pineda, Jun-Jie Wang

(Submitted on 17 Mar 2017)

https://arxiv.org/abs/1703.05864

We have obtained OH spectra of four transitions in the 2Π3/2 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∘, 64∘) and (189∘, 207∘) 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 1021 cm−2 and N (OH) ≥4.5×1015 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.