High Latitude, Translucent Molecular Clouds as Probes of Local Cosmic Rays

TL;DR

This study uses gamma-ray observations from the Fermi LAT to analyze high latitude molecular clouds, revealing insights into local cosmic ray densities, spectra, and the nature of dark gas in the interstellar medium.

Contribution

It provides the first detailed gamma-ray analysis of multiple high latitude clouds, showing their gamma-ray emission characteristics and implications for cosmic ray and dark gas properties.

Findings

Gamma-ray emission detected from all clouds.

No systematic variation in cosmic ray emissivity or spectral index.

Significant variation in CO-dark gas within clouds.

Abstract

We analyze the gamma-ray emission from 9 high latitude, translucent molecular clouds taken with the Fermi Large Area Telescope (LAT) between 250 MeV and 10 GeV. Observations of gamma-rays allow us to probe the density and spectrum of cosmic rays in the solar neighborhood. The clouds studied lie within $\sim\!$270 pc from the Sun and are selected from the Planck all-sky CO map. Gamma-rays in this energy range mostly result from cosmic ray interactions with the interstellar medium, which is traced with three components: HI, CO, and dark gas. Every cloud is detected and shows significant, extended gamma-ray emission from molecular gas. The gamma-ray emission is dominated by the CO-emitting gas in some clouds, but by the CO-dark gas in others. The average emissivity and gamma-ray power law index from HI above 1 GeV shows no evidence of a systematic variation. The CO-to-H$_2$ conversion factor shows no variation between clouds over this small spatial range, but shows significant variations within each cloud. The average CO-to-H$_2$ conversion factor suggests that the CO-dark gas is molecular as opposed to optically thick HI.