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Abstract:
The intersection between the atomic and molecular interstellar medium
(ISM) is still relatively mysterious. In the past two decades,
indirect gas tracers such as gamma-ray and dust emission have implied
the existence of abundant molecular hydrogen (H2) not traced by our
canonical molecular tracer, the CO molecule. This H2 likely lies in
diffuse clouds where CO will be not sufficiently collisionally excited
or even photodissociated. I will discuss recent efforts in using the
OH molecule in emission at 18cm to trace the large-scale Galactic dark
H2, and what we have learned about this previously invisible phase of
the diffuse molecular ISM through ultra-sensitive (RMS ~ 1mK) 18cm OH
emission surveys with the 100m Green Bank Telescope (GBT). Sensitive
OH observations serendipitously revealed an immense amount of dark H2
in the Outer Galaxy in the form of a diffuse disk, co-spatial with the
atomic phase as traced by the HI 21cm line. I will also discuss
upcoming GBT projects using the optically thin OH lines as a tool to
investigate Galactic structure and present the (preliminary) first
detection of thermal OH emission in another Galaxy (M31).
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