Is the Broad 18cm OH Emission ‘Disk’ in Concordance with Galactic Structure?
We recently published the GBT discovery paper of a large amount of diffuse molecular gas, previously undetected by CO surveys (Busch et al. 2021). Present in sightlines towards the Outer Galaxy was an extremely faint (~2-4 mK/channel) signal of 1667 MHz OH emission and corresponding 1665 MHz OH emission, typically 5/9 weaker, indicating quasi-LTE collisionally-dominated excitation. The signal was broad, and confirmation of the GBT spectrum was obtained using 100 hours of the GBO 20m telescope.
We wish to extend mapping of the longitude dependence of the OH emission, to determine if the emission behaves in velocity space as a large-scale Galactic structure. Does the emission experience velocity crowding at l = 180 degrees, and do the emission velocities become positive for l > 180 degrees? Measuring the longitude-velocity diagram of this new feature is of great astrophysical interest, as the terminal velocities will reveal whether the diffuse molecular phase of the gas disk is as extended as the atomic phase. We also wish to analyze uncertainties in baseline fits more accurately with the variety of spectra that this survey would provide.
Reduced Spectrum & Data Products
- Michael Busch
- Philip Engelke
Busch, Michael P., Engelke, Philip D., Allen, Ronald J., and Hogg, David E. (2021) “Observational Evidence for a Thick Disk of Dark Molecular Gas in the Outer Galaxy,” The Astrophysical Journal, 914:72