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

Abstract

Abstract We report a Fermi-LAT γ-ray analysis for the Chamaeleon molecular cloud complex using a total column density ( ) model based on the dust optical depth at 353 GHz ( ) with the Planck thermal dust emission model. Gamma rays with energy from 250 MeV to 100 GeV are fitted with the model as a function of , (α ≥ 1.0), to explicitly take into account a possible nonlinear / 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 model, we derived the CO-to- conversion factor ( ) and gas mass, taking into account the uncertainties of the model. The value of 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 , of which the mass of additional gas not traced by standard 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 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 and CO-dark . We also derived the γ-ray emissivity spectrum, which is consistent with the local emissivity derived from Fermi-LAT data within the systematic uncertainty of ∼20%.