# OH Survey along Sightlines of Galactic Observations of Terahertz C+

## TL;DR

This study presents OH spectral observations along multiple sightlines in the Galactic plane, revealing that OH traces molecular gas more effectively than CO in diffuse regions and providing insights into the structure of HI halos and molecular formation.

## Contribution

First comprehensive survey of OH in the Galactic plane linking OH, HI, and C+ emissions, highlighting OH as a superior tracer of diffuse molecular gas.

## Key findings

- OH emission obeys the 'Sum Rule', indicating optically thin conditions.
- OH column density correlates with HI halos, suggesting molecular formation processes.
- 18% of OH clouds are CO-dark but associated with C+ emission.

## 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) $\geq 4.5\times 10^{15}$ 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.