Résumé
The recent progress in H i, CO, dust, and γ-ray observations provides excellent opportunities toprobe the properties of the interstellar medium (ISM) at a resolution of a few parsecs inside nearbyclouds and to search for biases in the different gas tracers. The nearby clouds in the Galactic anticenter and Chamaleon regions have been studied using jointly the γ-ray observations of the FermiLarge Area Telescope, and the dust optical depth inferred from Planck and IRAS observations.We have quantified the potential variations in cosmic-ray density and dust properties across thedifferent gas phases and different clouds, and we have measured the CO-to-H2 conversion factor,XCO, in different environments. The measured interstellar γ-ray spectra support a uniform penetration of the cosmic rays with energies above a few GeV through the clouds, from the atomicenvelopes to the 12CO-bright cores. We find a gradual increase in dust opacity as the gas (atomicor molecular) becomes more dense which is likely caused by a chemical or structural change inthe dust grains. The XCO factors measured in γ rays show a decrease from diffuse to more compactmolecular clouds, as expected from theory. We also mapped the gas not seen, or poorly traced, byH i, free-free, and 12CO emissions, namely (i) the opaque H i and diffuse H2 present in the DarkNeutral Medium (DNM) at the atomic-molecular transition, and (ii) the dense H2 present where12CO lines saturate. We present these results showing how the precise modelling of the ISM wehave performed helps to better trace the total gas and so improve the modelling of the diffuseGalactic γ-ray emission of interstellar origin.