THE STRUCTURE OF DARK MOLECULAR GAS IN THE GALAXY. I. A PILOT SURVEY FOR 18 cm OH EMISSION TOWARD l 105°, b +1

• Published in: The Astronomical journal 2015-04, Vol.149 (4), p.123
• Main Authors: Allen, Ronald J., Hogg, David E., Engelke, Philip D.
• Format: Article
• Language: English
• Subjects: ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; EMISSION; GALAXIES; Galaxy: disk; GHZ RANGE 01-100; ISM: molecules; ISM: structure; local interstellar matter; MATTER; MOLECULES; radio lines: ISM; surveys
• ISSN: 0004-6256; 1538-3881; 1538-3881
• DOI: 10.1088/0004-6256/149/4/123

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 . This sparse, high-sensitivity survey ( mK rms in 0.55 km s−1 channels), was carried out as a pilot project with the Robert C. Byrd Green Bank Telescope (GBT) (FWHM ) on a 3 × 9 grid at spacing. The pointings were chosen to correspond with those of the existing 12CO(1-0) CfA survey of the Galaxy done at a similar resolution (8 4). With 2 hr integrations, 1667 MHz OH emission was detected with the GBT at of the 27 survey positions ( ), 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 wide 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 identified OH spectral features show detectable CO counterparts at the CfA sensitivity levels, and these are generally relatively faint. There are no CO features without corresponding OH emission in our survey. With very few exceptions, peaks in the OH profiles coincide with peaks in the GBT H i 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, although further work is needed to establish the quantitative details of the connection. Further aspects of the OH CO relation are revealed in a scatter plot of CO versus OH line strengths. This plot suggests a rough proportionality between the bright envelope of the CO emission (when present) and level of the accompanying 1667 MHz OH emission. Finally, we note several cases of anomalous OH emission. One survey position shows several narrow OH spectral features which are not well correlated with the H i spectrum; these features have been identified with a nearby known OH-IR star. Also, nine neighboring survey positions show enhanced emission at 1720 MHz, consistent with earlier observations and wit