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