Cosmic rays, gas and dust in nearby anticentre clouds - I. CO-to-H2 conversion factors and dust opacities

摘要

Aims. We aim to explore the capabilities of dust emission and γ rays for probing the properties of the interstellar medium in the nearby anti-centre region, using γ -ray observations with the Fermi Large Area Telescope (LAT), and the thermal dust optical depth inferred from Planck and IRAS observations. We also aim to study massive star-forming clouds including the well known Taurus, Auriga, Perseus, and California molecular clouds, as well as a more diffuse structure which we refer to as Cetus. In particular, we aim at quantifying potential variations in cosmic-ray density and dust properties per gas nucleon across the different gas phases and different clouds, and at measuring the CO-to-H _(2) conversion factor, X _(CO) , in different environments. Methods. We have separated six nearby anti-centre clouds that are coherent in velocities and distances, from the Galactic-disc background in H? i 21-cm and ~(12) CO 2.6-mm line emission. We have jointly modelled the γ -ray intensity recorded between 0.4 and 100 GeV, and the dust optical depth τ _(353) at 353 GHz as a combination of H? i -bright, CO-bright, and ionised gas components. The complementary information from dust emission and γ rays was used to reveal the gas not seen, or poorly traced, by H? i , free-free, and ~(12) CO emissions, namely (i) the opaque H? iand diffuse H _(2) present in the Dark Neutral Medium at the atomic-molecular transition, and (ii) the dense H _(2) to be added where ~(12) CO lines saturate. Results. 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 ~(12) CO-bright cores, and with a small ± 9 % cloud-to-cloud dispersion in particle flux. We detect the ionised gas from the H? iiregion NGC 1499 in the dust and γ -ray emissions and measure its mean electron density and temperature. We find a gradual increase in grain opacity as the gas (atomic or molecular) becomes more dense. The increase reaches a factor of four to six in the cold molecular regions that are well shielded from stellar radiation. Consequently, the X _(CO) factor derived from dust is systematically larger by 30% to 130% than the γ -ray estimate. We also evaluate the average γ -ray X _(CO) factor for each cloud, and find that X _(CO) tends to decrease from diffuse to more compact molecular clouds, as expected from theory. We find X _(CO) factors in the anti-centre clouds close to or below 10~(20) cm ~(-2) ?K ~(-1) ?km ~(-1) ?s, in agreement with other estimates in the solar neighbourhood. Together, they confirm the long-standing unexplained discrepancy, by a factor of two, between the mean X _(CO) values measured at parsec scales in nearby clouds and those obtained at kiloparsec scale in the Galaxy. Our results also highlight large quantitative discrepancies in ~(12) CO intensities between simulations and observations at low molecular gas densities.