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

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.

arXiv commentAccepted for publication in ApJ. 20 pages, 11 figures, 7 tables

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• Calibrating Column Density Tracers with Gamma-ray Observations of the $\rho$ Ophiuchi Molecular Cloud
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