Cosmic rays, gas, and dust in local clouds
Résumé
The recent progress in H i, CO, dust, and γ-ray observations provides excellent opportunities to
probe the properties of the interstellar medium (ISM) at a resolution of a few parsecs inside nearby
clouds 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 Fermi
Large 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 the
different 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 atomic
envelopes to the 12CO-bright cores. We find a gradual increase in dust opacity as the gas (atomic
or molecular) becomes more dense which is likely caused by a chemical or structural change in
the dust grains. The XCO factors measured in γ rays show a decrease from diffuse to more compact
molecular clouds, as expected from theory. We also mapped the gas not seen, or poorly traced, by
H i, free-free, and 12CO emissions, namely (i) the opaque H i and diffuse H2 present in the Dark
Neutral Medium (DNM) at the atomic-molecular transition, and (ii) the dense H2 present where
12CO lines saturate. We present these results showing how the precise modelling of the ISM we
have performed helps to better trace the total gas and so improve the modelling of the diffuse
Galactic γ-ray emission of interstellar origin.