Molecular Clouds as Cosmic-Ray Barometers

TL;DR

This paper discusses how molecular clouds can serve as natural detectors to measure cosmic-ray flux variations across the Galaxy using gamma-ray observations.

Contribution

It introduces a new methodology to estimate gamma-ray signals from molecular clouds for testing cosmic-ray flux distribution in the Galaxy.

Findings

New data on molecular hydrogen distribution from NANTEN survey.

A methodology to compare gamma-ray observations with cosmic-ray flux models.

Potential to map cosmic-ray flux variations using molecular clouds.

Abstract

The advent of high sensitivity, high resolution gamma-ray detectors, together with a knowledge of the distribution of the atomic hydrogen and especially of the molecular hydrogen in the Galaxy on sub-degree scales creates a unique opportunity to explore the flux of cosmic rays in the Galaxy. We here present the new data on the distribution of the molecular hydrogen from a large region of the inner Galaxy obtained by the NANTEN Collaboration. We then introduce a methodology which aims to provide a test bed for current and future gamma-ray observatories to explore the cosmic ray flux at various positions in our Galaxy. In particular, for a distribution of molecular clouds, as provided by the NANTEN survey, and local cosmic ray density as measured at the Earth, we estimate the expected GeV to TeV gamma-ray signal, which can then be compared with observations and use to test the cosmic ray flux.