HyGAL is a SOFIA legacy project (US-PI David Neufeld, German PI Peter Schilke) that aims at determining the composition of the interstellar medium by observing hydride and other small molecules molecules.  This data were taken in 2021 and 2022. 

By means of absorption-line spectroscopy towards 22 background terahertz continuum sources widely distributed within the Galactic plane, we will obtain robust measurements of the column densities of six hydride molecules (OH⁺, H₂O⁺, ArH⁺, SH, OH and CH) and two key atomic constituents (C⁺ and O) within the diffuse ISM. Recent studies with Herschel have demonstrated the unique value of specific hydride molecules as quantitative diagnostic probes of the H₂ fraction, the cosmic-ray ionization rate, or of “warm chemistry” associated with the dissipation of interstellar turbulence in regions of elevated temperature or ion-neutral drift.  These observations will allow us to address several related questions:

1. What is the distribution function of H₂ fraction in the ISM?
2. How does the density of low-energy cosmic-rays vary within the Galaxy?
3. What is the nature of interstellar turbulence (e.g. typical shear or shock velocities), and what mechanisms lead to its dissipation?

Because of atmospheric absorption, the transitions to be observed in this program are inaccessible from the ground and can only be observed from airborne or satellite observatories. Our investigation is synergistic with ancillary observations of non-hydride molecules that have been/will be performed with ALMA, JVLA, the IRAM 30 m telescope, and APEX. There is a very timely synergy with theoretical models for H₂ formation within the turbulent ISM, several of which are under development by groups within our team.

The anticipated results are

1. a determination of the distribution function for the H₂ fraction in the Galaxy, and how it varies;
2. a determination of the cosmic-ray ionization rate and how it varies;
3. an improved characterization of turbulence in the diffuse ISM, and its dissipation;
4. the provision of enhanced data products that will serve as a legacy for future ISM studies

Two HyGAL papers have been published so far (see also Publications)

1. HyGAL. I. Survey Description and a First Look Toward W3(OH), W3 IRS5, and NGC 7538 IRS1, by Jacob, A. M., Neufeld, D. A., Schilke, P. et al 2022, ApJ, 930, 141
2. HyGAL. II. The absorption line survey with the IRAM 30 m telescope, by Kim, W.-J., Schilke, P., Neufeld, D. A., et al. 2023, A&A in press (arXiv:2212.09334)