GBT24A-144

Probing the structure of the diffuse molecular ISM--mixed or clumped?

Abstract

The discovery of an ultra-diffuse disk of OH emission in the Outer Galaxy with the GBT remains an enduring puzzle (Busch 2021). What is the structure of the molecule-bearing gas within the GBT beam? Is it organised into dense cloudlets, or is it more uniformly mixed throughout the ISM? Intriguingly, MHD simulations suggest that molecular gas can form in dense regions and diffuse out into the or warm or "lukewarm" neutral medium, surviving for up to 10^6 years (Godard et al. 2023). The recent discovery of broad HCO+ absorption features (Rybarczyk 2022), analogous to the GBT broad OH emission, may allow us to test this possibility. We propose to observe OH towards three sources in that paper. Comparisons between the pencil-beam HCO+ absorption and beam-smoothed OH emission will enable us to assess whether the molecule-bearing gas is contained in isolated cloudlets or in smoother structures mixed throughout the ISM. We will compare spectral structure by using Gaussian and Fourier decomposition and will also compare with HI emission and absorption spectra, to understand the relationship between the diffuse molecule-bearing gas, the CNM and the WNM.

Investigators

Name	Institution
GBT Operator	Green Bank Observatory
Michael Busch *	California at San Diego, University of
Joanne Dawson	Macquarie University; Commonwealth Scientific and Industrial Research Organisation
Daniel Rybarczyk	Wisconsin at Madison, University of
Harvey Liszt	National Radio Astronomy Observatory; National Institute of Flamenco; Belize, Pontifical University of
Maryvonne Gerin	Paris, Observatoire de

* indicates the PI