Fingerprinting the chemical Universe: A tale of GREAT hydride observations

Owing to the complexity of their energy level structures and as a result their chemical fingerprints or spectra– molecules, observed in either emission or absorption form unique diagnostic probes of the physical conditions in which they are found. In addition to forming excellent probes of astronomical conditions, molecules play a central role in influencing the thermal and ionization structure of gas, initiating condensations and instabilities responsible for the formation of stars and as a result regulate the evolution of the environments in which they are formed. While astronomical spectra reveal a wealth of knowledge, their interpretation is complicated by the need for a detailed understanding of their chemistry and often limited to the study of simpler systems such as light hydrides (molecules/molecular ions with a heavy atom covalently bonded to one or more hydrogen atoms).

In this talk, I will highlight the use of hydrides in addressing one of the fundamental questions in modern astronomy that concerns the life cycle of molecular material in the universe. I will emphasis HyGAL, a SOFIA-GREAT Legacy program aimed to address several questions related to the HI-to-H₂ phase transition (responsible for molecular cloud formation) and star-formation in general using observations of hydrides, such as (1) What is the distribution of the H₂ fraction in the ISM? (2) How does the density of low-energy cosmic-rays vary within in the Galaxy and (3) What is the nature of interstellar turbulence and what mechanisms lead to dissipation? I will conclude by discussing the valuable legacy provided by HyGAL for future studies of the ISM and the role it plays in future hydride observations.