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Michael Busch (Johns Hopkins University)

Michael Busch Colloquium: Revealing the Galactic Dark H2 with OH

The Australia Telescope National Facility Colloquium
11:00-12:00 Wed 02 Jun 2021



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).


Andrew Zic

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