Fermi-LAT γ-Ray Study of the Interstellar Medium and Cosmic Rays in the Chamaeleon Molecular Cloud Complex: A Look at the Dark Gas as Optically Thick H I

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 ({τ }₃₅₃) 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 {τ }₃₅₃, {N}_{{H}}\propto {τ }₃₅₃^1/α (α ≥ 1.0), to explicitly take into account a possible nonlinear {τ }₃₅₃/{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-{{{H}}}₂ 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) ×10²⁰ 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) × 10⁴ {M}_⊙ , of which the mass of additional gas not traced by standard {{H}} {{I}} 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 {{H}} {{I}} 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 {{H}} {{I}} and CO-dark {{{H}}}₂. We also derived the γ-ray emissivity spectrum, which is consistent with the local {{H}} {{I}} emissivity derived from Fermi-LAT data within the systematic uncertainty of ∼20%.