Cosmic-rays, gas, and dust in nearby anti-centre clouds. III. Dust extinction, emission, and grain properties

Aims: We have explored the capabilities of dust extinction and γ rays to probe the properties of the interstellar medium in the nearby anti-centre region. In particular, we aim at quantifying the variations of the dust properties per gas nucleon across the different gas phases and different clouds. The comparison of dust extinction and emission properties with other physical quantities of large grains (emission spectral index β, dust colour temperature T_dust, total-to-selective extinction factor R_V) helps the theoretical modelling of grains as they evolve from diffuse to dense cloud environments.
Methods: We have jointly modelled the γ-ray intensity, recorded between 0.4 and 100 GeV with the Fermi Large Area Telescope (LAT), and the stellar reddening, E(B - V), inferred from Pan-STARRS and 2MASS photometry, as a combination of HI-bright, CO-bright, and ionised gas components. The complementary information from dust reddening and γ rays is used to reveal the gas not seen, or poorly traced, by HI, free-free, and ¹²CO emissions, namely (i) the opaque HI and diffuse H₂ present in the dark neutral medium (DNM) at the atomic-molecular transition, and (ii) the dense H₂ to be added where ¹²CO lines saturate (CO_sat). We compare the total gas column densities, N_H, derived from the γ rays and stellar reddening with those inferred from a similar, previously published analysis of γ rays and of the optical depth of the thermal dust emission, τ₃₅₃, at 353 GHz. We can therefore compare environmental variations in specific dust reddening, E(B - V)/N_H, and in dust emission opacity (dust optical depth per gas nucleon), τ₃₅₃/N_H.
Results: The gas column densities obtained when combining γ rays with either dust reddening or dust emission compare reasonably well in the atomic and DNM gas phases and over most of the CO-bright phase, but we find localised differences in the dense media (CO_sat component) due to differences in the two dust tracers. Over the whole anti-centre region, we find an average E(B - V)/N_H ratio of (2.02 ± 0.48) ×10^-22 mag cm², with maximum local variations of about ± 30% at variance with the two to six fold coincident increase seen in emission opacity as the gas column density increases. We show how the specific reddening and opacity vary with the colour temperature and spectral index of the thermal emission of the large grains. Additionally, we find a better agreement between the X_CO = N(H₂)/W_CO conversion factors derived with dust reddening or with γ rays than with those inferred from dust emission, especially towards clouds with large τ₃₅₃ optical depths. The comparison confirms that the high X_CO values found with dust emission are biased by the significant rise in emission opacity inside molecular clouds.
Conclusions: In the diffuse medium, we find only small variations in specific reddening, E(B - V)/N_H, compatible with the dispersion in the R_V factor reported by other studies. This implies a relatively uniform dust-to-gas mass ratio in the diffuse parts of the anti-centre clouds. The small amplitude of the E(B - V)/N_H variations with increasing N_H column density confirms that the large opacity τ₃₅₃/N_H rise seen towards dense CO clouds is primarily due to changes in dust emissivity. The environmental changes are qualitatively compatible with model predictions based on mantle accretion on the grains and the formation of grain aggregates.