# OH as an Alternate Tracer for Molecular Gas: Quantity and Structure of Molecular Gas in W5

## TL;DR

This study uses OH 18-cm emission to trace molecular gas in W5, revealing that OH can detect more gas than CO and providing insights into the gas structure and mass estimates in star-forming regions.

## Contribution

It demonstrates that OH can serve as an alternative tracer for molecular gas, especially in cases where CO may underestimate the gas content, and models the small-scale structure of OH clouds.

## Key findings

- OH traces greater column densities than CO in some cases.
- Mass of molecular gas from OH is roughly twice that from CO.
- Filling factor of OH gas is estimated to be a few to 10 percent.

## Abstract

We report column densities of molecular gas in the W5 star-forming region as traced with OH 18-cm emission in a grid survey using the Green Bank Telescope. OH appears to trace a greater column density than does CO in 8 out of 15 cases containing OH emission detections; the two molecules trace the same column densities for the other 7 cases. OH and CO trace a similar morphology of molecular gas with a nearly one-to-one correspondence. The mass of molecular gas traced by OH in the portion of the survey containing OH emission is $1.7$ (+ 0.6 or - 0.2) $\times 10^4 M_{\odot}$, whereas the corresponding CO detections trace $9.9 \times 10^3 M_{\odot} (\pm 0.7) \times 10^3$. We find that for lines observed in absorption, calculations assuming uniform gas and continuum distributions underestimate column density values by 1 to 2 orders of magnitude, making them unreliable for our purposes. Modeling of this behavior in terms of OH cloud structure on a scale smaller than telescopic resolution leads us to estimate that the filling factor of OH gas is a few to 10 percent. Consideration of filling factor effects also results in a method of constraining the excitation temperature values. The total molecular gas content of W5 may be approximately two to three times what we report from direct measurement, because we excluded absorption line detections from the mass estimate.