Cosmic-ray and Interstellar Gas Properties in the Solar Neighborhood Revealed by Diffuse Gamma Rays

TL;DR

This study uses Fermi LAT gamma-ray data to analyze the interstellar medium and cosmic ray properties in five nearby molecular cloud regions, revealing detailed gas components and cosmic ray gradients in the solar neighborhood.

Contribution

It provides a detailed decomposition of interstellar gas into various components using gamma-ray data, and compares cosmic ray intensities with models based on local measurements.

Findings

Cosmic ray intensities increase toward the inner Galaxy by about 10%.

The ratio of CO-dark to CO-bright H2 varies with molecular mass, reaching 5-10 in small systems.

Gamma-ray data successfully decomposes interstellar gas into multiple components.

Abstract

To investigate the interstellar medium (ISM) and Galactic cosmic rays (CRs) in the solar neighborhood, we analyzed ${\gamma}$-ray data by Fermi Large Area Telescope (LAT) for five nearby molecular cloud regions. Our data includes the MBM/Pegasus region (MBM~53, 54, 55 clouds and Pegasus loop), R CrA region (R Coronae Australis clouds), Chamaeleon region (Chamaeleon clouds), Cep/Pol region (Cepheus and Polaris flare), and Orion region (Orion clouds). The ISM templates are constructed by a component decomposition of the 21~cm {\HI} line, the Planck dust emission model, and the carbon monoxide (CO) 2.6~mm line. Through $\gamma$-ray data analysis the ISM gas is successfully decomposed into non-local {\HI}, narrow-line and optically thick {\HI}, broad-line and optically thin {\HI}, CO-bright {\Htwo}, and CO-dark {\Htwo} for all five regions. CR intensities evaluated by the ${\gamma}$-ray emissivity of broad {\HI} agree well with a model based on directly-measured CR spectra at the Earth, with a gradient giving a higher CR intensity toward the inner Galaxy at the 10\% level in ${\sim}$ 500~pc. The ratio of CO-dark {\Htwo} to CO-bright {\Htwo} anti-correlates with the {\Htwo} mass traced by the CO 2.6~mm line, and reaches 5--10 for small systems of ${\sim}$1000 solar mass.