The Structure of Dark Molecular Gas in the Galaxy - I: A Pilot Survey
  for 18-cm OH Emission Towards $l \approx 105^{\deg}, b \approx +1^{\deg}$
  We report the first results from a survey for 1665, 1667, and 1720 MHz OH
emission over a small region of the Outer Galaxy centered at $l \approx
105.0\deg , b \approx +1.0\deg$ . This sparse, high-sensitivity survey ($\Delta
Ta \approx \Delta Tmb \approx 3.0 - 3.5$ mK rms in 0.55 km/s channels), was
carried out as a pilot project with the Green Bank Telescope (GBT, FWHM
$\approx 7.6'$) on a 3 X 9 grid at $0.5\deg$ spacing. The pointings chosen
correspond with those of the existing $^{12}$CO(1-0) CfA survey of the Galaxy
(FWHM $\approx 8.4'$). With 2-hr integrations, 1667 MHz OH emission was
detected with the GBT at $\gtrsim 21$ of the 27 survey positions ($\geq 78\%$
), confirming the ubiquity of molecular gas in the ISM as traced by this
spectral line. With few exceptions, the main OH lines at 1665 and 1667 MHz
appear in the ratio of 5:9 characteristic of LTE at our sensitivity levels. No
OH absorption features are recorded in the area of the present survey, in
agreement with the low levels of continuum background emission in this
direction. At each pointing the OH emission appears in several components
extending over a range of radial velocity and coinciding with well-known
features of Galactic structure such as the Local Arm and the Perseus Arm. In
contrast, little CO emission is seen in the survey area; less than half of the
$\gtrsim 50$ identified OH components show detectable CO at the CfA sensitivity
levels, and these are generally faint. There are no CO profiles without OH
emission. With few exceptions, peaks in the OH profiles coincide with peaks in
the GBT HI spectra (obtained concurrently, FWHM $8.9'$), although the converse
is not true. We conclude that main-line OH emission is a promising tracer for
the "dark molecular gas" in the Galaxy discovered earlier in Far-IR and
gamma-ray emission. Further work is needed to establish the quantitative
details of this connection.
