Cold Clouds as Cosmic-Ray Detectors

TL;DR

This paper proposes a novel method to measure low energy cosmic-ray ionization rates in interstellar clouds using H$_2$ rovibrational line emissions excited by cosmic rays, potentially improving understanding of cosmic-ray spectra and origins.

Contribution

It introduces a new observational technique to constrain cosmic-ray ionization rates via H$_2$ line emissions excited by cosmic rays, complementing existing indirect methods.

Findings

CR-excited H$_2$ lines can be detected with VLT over 8 hours

Method can improve constraints on low energy cosmic-ray spectra

Potential to study cosmic-ray origins across different Galactic regions

Abstract

Low energy cosmic-rays (CRs) are responsible for gas heating and ionization of interstellar clouds, which in turn introduces coupling to Galactic magnetic fields. So far the CR ionization rate (CRIR) has been estimated using indirect methods, such as its effect on the abundances of various rare molecular species. Here we show that the CRIR may be constrained from line emission of H$_2$ rovibrational transitions, excited by CRs. We derive the required conditions for CRs to dominate line excitation, and show that CR-excited lines may be detected with the Very Large Telescope (VLT) over 8 hours integration. Our method, if successfully applied to a variety of clouds at different Galactic locations will provide improved constraints on the spectrum of low energy CRs and their origins.