OH as a Tracer of “CO-Dark” Gas in the Interstellar Medium Near and Far

Abstract:

Molecular clouds are the birthplace of stars. It is unfortunate then that the most abundant interstellar molecule, molecular hydrogen (H2), is practically invisible in cold molecular clouds (T<100K). Astronomers typically use the second most abundant molecule, carbon monoxide (CO), to trace the bulk distribution of H2 in our galaxy and many others because of its relative brightness and abundance. CO observations alone, however, fail to trace a significant amount of molecular gas, known as “CO-dark” molecular gas. In this talk, I will review the observational evidence for the existence of a significant amount of “CO-dark” gas in the galaxy, then recount the efforts in the 2010s to use OH with the Green Bank Telescope (GBT) to observe the “CO-dark” gas in the Outer galaxy (Busch et al. 2019, 2021). Then I will present recent results on an extremely sensitive pilot survey for 18cm OH emission towards the southern disk of the Andromeda Galaxy, in which trace the large-scale galactic “CO-Dark” molecular gas with the GBT (Busch 2024). A follow-up Andromeda survey is presented. I will discuss recent efforts in combining HCO+ absorption in the diffuse ISM with OH emission to understand the properties of diffuse molecular gas in the ISM and conclude on future work with collaborators in Australia regarding OH as a tracer of “CO-dark” gas.

Location

Organiser