2025PASJ...77..965M

To investigate the interstellar medium (ISM) and Galactic cosmic rays (CRs) in the solar neighborhood, we analyzed γ-ray data by the Fermi Large Area Telescope (LAT) for five nearby molecular cloud regions. Our data include the MBM/Pegasus (MBM 53, 54, 55 clouds and Pegasus loop), R CrA (R Coronae Australis clouds), Chamaeleon (Chamaeleon clouds), Cep/Pol (Cepheus and Polaris flare), and Orion (Orion clouds) regions. The ISM templates are constructed by a component decomposition of the 21 cm H I line, the Planck dust emission model, and the carbon monoxide (CO) 2.6 mm line. Through γ-ray data analysis the ISM gas is successfully decomposed into non-local H I, narrow-line and optically thick H I, broad-line and optically thin H I, CO-bright H₂, and CO-dark H₂ for all five regions. CR intensities evaluated by the γ-ray emissivity of broad H I 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 ∼500 pc. The ratio of CO-dark H₂ to CO-bright H₂ anticorrelates with the H₂ mass traced by the CO 2.6 mm line, and reaches 5-10 for small systems of ∼1000 solar mass.