HyGAL: Characterizing the Galactic ISM with observations of hydrides and other small molecules -- I. Survey description and a first look toward W3(OH), W3 IRS5 and NGC 7538 IRS1

Abstract

The HyGAL SOFIA legacy program surveys six hydride molecules -ArH + , OH + , H 2 O + , 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 lifecycle 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 cosmicrays 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.

Photodissociation and X-Ray Dominated Regions

“…In addition to the inferred H2 fractions, observations have directly detected molecules in diffuse molecular clouds in absorption e.g., HCO + , HCN (Hogerheijde et al 1995a, HF, H2O (Flagey et al 2013, Sonnentrucker et al 2015, OH + , H2O + (e.g., Wyrowski et al 2010, Gerin et al 2010, ArH + (Schilke et al 2014, Jacob et al 2020, CH Sheffer et al (2008), SH Neufeld et al (2015), and in both absorption and emission e.g., OH , Busch et al 2019, with CH, HF and H2O being particularly good (linear) tracers of the H2 fraction. Although not strictly CO-dark in the original sense since the bulk of the gas may be detectable in H I, nevertheless these observations detect a portion of molecular gas that is not seen in CO emission.…”

“…Observations of molecular ions have been used to constrain the cosmic-ray ionization rate in diffuse gas (see Section 3.3). See also the HyGAL survey using SOFIA/GREAT (Jacob et al 2022), and reviews by Snow & McCall (2006) and Gerin et al (2016) for additional observations and models.…”