2018A&A...611A..51R

Aim. HI 21-cm and ¹²CO 2.6-mm line emissions trace the atomic and molecular gas phases, respectively, but they miss most of the opaque HI and diffuse H₂ present in the dark neutral medium (DNM) at the transition between the HI-bright and CO-bright regions. Jointly probing HI, CO, and DNM gas, we aim to constrain the threshold of the HI-H₂ transition in visual extinction, A_V, and in total hydrogen column densities, N_H^tot. We also aim to measure gas mass fractions in the different phases and to test their relation to cloud properties.
Methods. We have used dust optical depth measurements at 353GHz, γ-ray maps at GeV energies, and HI and CO line data to trace the gas column densities and map the DNM in nearby clouds toward the Galactic anticentre and Chamaeleon regions. We have selected a subset of 15 individual clouds, from diffuse to star-forming structures, in order to study the different phases across each cloud and to probe changes from cloud to cloud.
Results. The atomic fraction of the total hydrogen column density is observed to decrease in the (0.6-1)x10²¹cm^–2 range in N_H^tot (A_V~=0.4mag) because of the formation of H₂ molecules. The onset of detectable CO intensities varies by only a factor of 4 from cloud to cloud, between 0.6x10²¹cm^–2 and 2.5x10²¹cm^–2 in total gas column density. We observe larger H₂ column densities than linearly inferred from the CO intensities at A_V>3mag because of the large CO optical thickness; the additional H₂ mass in this regime represents on average 20% of the CO-inferred molecular mass.In the DNM envelopes, we find that the fraction of diffuse CO-dark H₂ in the molecular column densities decreases with increasing A_V in a cloud. For a half molecular DNM, the fraction decreases from more than 80% at 0.4mag to less than 20% beyond 2 mag.In mass, the DNM fraction varies with the cloud properties. Clouds with low peak CO intensities exhibit large CO-dark H₂ fractions in molecular mass, in particular the diffuse clouds lying at high altitude above the Galactic plane. The mass present in the DNM envelopes appears to scale with the molecular mass seen in CO as M_H(DNM)=62±7M_H2(CO^0.51±0.02) across two decades in mass.
Conclusions. The phase transitions in these clouds show both common trends and environmental differences. These findings will help support the theoretical modelling of H₂ formation and the precise tracing of H₂ in the interstellar medium.