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 (84). 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 89), 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 with models involving extended regions of elevated particle density.
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机译：我们报告了一项以1665、1667和1720 MHz OH发射为中心的小区域外的调查的第一结果。这项稀疏的高灵敏度测量（0.55 km s？1通道中的mK rms）是作为试点项目，由Robert C. Byrd Green Bank望远镜（GBT）（FWHM）在3×9的网格上以一定间隔进行的。选择的指示与以类似分辨率进行的现有的银河系12CO（1-0）CfA调查的指示相对应（84）。经过2小时的积分，在27个测量位置（）处的GBT处检测到1667 MHz的OH发射，证实了该谱线所追踪的ISM中分子气体的普遍存在。除少数例外，在我们的灵敏度水平上，1665和1667 MHz的主要OH线以LTE特性的5：9比率出现。在本次调查的区域中，未记录任何OH吸收特征，这与该方向上连续背景发射的水平较低相符。在每个指向上，OH发射都出现在几个组件上，这些组件在广泛的径向速度范围内延伸，并与银河结构的众所周知的特征（例如本地臂和英仙臂）相吻合。相反，在调查区域内几乎看不到一氧化碳排放。不到一半的已识别OH光谱特征在CfA灵敏度水平上显示出可检测到的CO对应物，并且通常相对较弱。在我们的调查中，没有相应的OH排放没有CO特征。除少数情况外，OH谱中的峰与GBT H i谱中的峰重合（同时获得，FWHM 89），尽管事实并非如此。我们得出的结论是，尽管在确定连接的定量细节方面还需要做更多的工作，但主线OH的排放是较早发现的银河中“暗分子气体”的有希望的示踪剂。 OH CO关系的其他方面在CO对OH线强度的散点图中显示。该图表明了CO排放的亮包络线（如果存在）与伴随的1667 MHz OH排放水平之间的大致比例关系。最后，我们注意到几种OH排放异常的情况。一个调查位置显示了几个狭窄的OH光谱特征，这些特征与H i光谱没有很好的相关性。这些特征已被附近的已知OH-IR星识别。此外，九个相邻的测量位置显示出在1720 MHz处发射增强，这与早期的观察结果以及涉及扩展的具有较高粒子密度区域的模型相一致。
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