Distribution of Cosmic-Ray Ionization Rates in the Galactic Diffuse Interstellar Medium as Inferred from Observations of H$_{3}^{+}$, OH⁺, and H₂O⁺

Abstract

Cosmic rays play a vital role in initiating the chemistry that occurs in molecular clouds. The ionization of H and H₂ begins a network of ion-molecule reactions responsible for generating many of the diatomic and small polyatomic molecules observed in the ISM. A few such species—OH⁺, H₂O⁺, and H$_{3}^{+}$ in particular—are formed and destroyed by rather simple processes, making them powerful probes of the cosmic-ray ionization rate. At present, we have performed observations searching for H$_{3}^{+}$ absorption in over 50 sight lines, and for OH⁺ and H₂O⁺ absorption in an additional 8 sight lines. Using these observations, we have inferred the distribution of cosmic-ray ionization rates in the diffuse ISM throughout the Galaxy. Some of the highest ionization rates are about 25 times larger than the lowest upper limits, suggesting variations in the underlying low-energy cosmic-ray flux across the Galaxy. We posit that such variations are caused by the distance between an observed cloud and the nearest site of particle acceleration, a conjecture supported by the high ionization rates found in close proximity to supernova remnants.