2019ApJ...884..130H

We report a Fermi-LAT γ-ray analysis for the Chamaeleon molecular cloud complex using a total column density (N_H) model based on the dust optical depth at 353 GHz (τ₃₅₃) with the Planck thermal dust emission model. Gamma rays with energy from 250 MeV to 100 GeV are fitted with the N_H model as a function of τ₃₅₃, N_H∝τ₃₅₃^1/α^ (α >= 1.0), to explicitly take into account a possible nonlinear τ₃₅₃/N_H ratio. We found that a nonlinear relation, α ∼ 1.4, gives the best fit to the γ-ray data. This nonlinear relation may indicate dust evolution effects across the different gas phases. Using the best-fit N_H model, we derived the CO-to-H₂ conversion factor (X_CO) and gas mass, taking into account the uncertainties of the N_H model. The value of X_CO is found to be (0.63-0.76) x10²⁰ cm^–2 K^–1 km^–1 s, which is consistent with that of a recent γ-ray study of the Chamaeleon region. The total gas mass is estimated to be (6.0-7.3) x 10⁴ M_☉, of which the mass of additional gas not traced by standard HI or CO line surveys is 20%-40%. The additional gas amounts to 30%-60% of the gas mass estimated in the case of optically thin HI and has five to seven times greater mass than the molecular gas traced by CO. Possible origins of the additional gas are discussed based on scenarios of optically thick HI and CO-dark H₂. We also derived the γ-ray emissivity spectrum, which is consistent with the local HI emissivity derived from Fermi-LAT data within the systematic uncertainty of ∼20%.