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
  The HyGAL SOFIA 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 lifecycle of molecular material in the ISM and the physical
processes that impact its phase transition, such as: (1) What is the
distribution function of the H2 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, and what mechanisms lead to its
dissipation? This overview discusses the observing strategy, synergies with
ancillary and archival observations, the data reduction and analysis schemes
adopted; and presents the first results obtained toward three of the survey
targets, W3(OH), W3IRS5 and NGC7538IRS1. 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 timely
synergy between these observations and the development of theoretical models,
particularly pertaining to the formation of H2 within the turbulent ISM. The
provision of enhanced HyGAL data products will therefore serve as a legacy for
future ISM studies.
