HyGAL: Characterizing the Galactic Interstellar Medium with Observations of Hydrides and Other Small Molecules. I. Survey Description and a First Look Toward W3(OH), W3 IRS5, and NGC 7538 IRS1

The HyGAL Stratospheric Observatory for Infrared Astronomy legacy program surveys six hydride molecules-ArH+, OH+, H2O+, SH, OH, and CH-and two atomic constituents-C+ and O-within the diffuse interstellar medium (ISM) by means of absorption-line spectroscopy toward 25 bright Galactic background continuum sources. This detailed spectroscopic study is designed to exploit the unique value of specific hydrides as tracers and probes of different phases of the ISM, as demonstrated by recent studies with the Herschel Space Observatory. The observations performed under the HyGAL program will allow us to address several questions related to the life cycle of molecular material in the ISM and the physical processes that impact the phase transition from atomic to molecular gas, such as: (1) What is the distribution function of the H-2 fraction in the ISM? (2) How does the ionization rate due to 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? In this overview, we discuss the observing strategy, the synergies with ancillary and archival observations of other small molecules, and the data reduction and analysis schemes we adopted; and we present the first results obtained toward three of the survey targets, W3(OH), W3 IRS5, and NGC 7538 IRS1. Robust measurements of the column densities of these hydrides-obtained through widespread observations of absorption lines-help address the questions raised, and there is a very timely synergy between these observations and the development of theoretical models, particularly pertaining to the formation of H-2 within the turbulent ISM. The provision of enhanced HyGAL data products will therefore serve as a legacy for future ISM studies.