The Structure of Dark Molecular Gas in the Galaxy - I: A Pilot Survey for 18-cm OH Emission Towards $l\approx 105^{°},b\approx +1^{°}$

Abstract

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 $l\approx 105.0°,b\approx +1.0°$ . 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 $\approx 7.6^{\prime }$) on a 3 X 9 grid at $0.5°$ spacing. The pointings chosen correspond with those of the existing ${}^{12}$CO(1-0) CfA survey of the Galaxy (FWHM $\approx 8.4^{\prime }$). With 2-hr integrations, 1667 MHz OH emission was detected with the GBT at $\gtrsim 21$ of the 27 survey positions ($\ge 78\%$ ), 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 $8.9^{\prime }$), 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.

24 pages, 10 figures, accepted for publication in the Astronomical Journal on 1/30/15

References in corpus (4)

• Trigonometric Parallaxes of Star Forming Regions in the Perseus Spiral Arm
• A Survey of Hydroxyl Toward Supernova Remnants: Evidence for Extended 1720 MHz Maser Emission
• GBT Observations of IC 443: the Nature of OH(1720 MHz) Masers and OH Absorption
• Excited-state OH Masers and Supernova Remnants

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• Sensitive CO(1-0) Survey in Pegasus-Pisces Reduces CO-Dark Gas Inventory by Factor of Two
• OH as an Alternate Tracer for Molecular Gas: Quantity and Structure of Molecular Gas in W5
• The Interstellar Medium: The Key Component in Galactic Evolution and Modern Cosmology
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