Cold Clouds as Cosmic-Ray Detectors

Shmuel Bialy

Published 2019-10-28Version 1

We show that the cosmic-ray (CR) ionization rate (CRIR) may be constrained by observations of ro-vibrational H$_2$ lines in cold and dense molecular clouds. The H$_2$ is excited by penetrating electrons produced by CR ionization resulting in line emission that is proportional to the CRIR. The strongest CR excited lines are (1-0)O(2), S(0), Q(2) and O(4) in the 2-3 $\mu$m range. We derive an analytic framework for line formation by various pumping mechanisms: CR, ultraviolet (UV), and H$_2$-formation pumping, as functions of the CRIR and the UV field intensity. We obtain the required conditions for CRs to dominate line excitation, and find that the S(0) line may be detected with X-shooter instrument on the Very Large Telescope over one observation night. This new method, if successfully applied to a variety of clouds at different Galactic locations and with varying gas columns, will provide improved constraints on the spectrum of low energy CRs, and their origins.