2015ApJ...809...56L

We investigate the impact of high optical depth on the HI saturation observed in the Perseus molecular cloud by using Arecibo HI emission and absorption measurements toward 26 radio continuum sources. The spin temperature and optical depth of individual HI components are derived along each line of sight, enabling us to estimate the correction for high optical depth. We examine two different methods for the correction, Gaussian decomposition and isothermal methods, and find that they are consistent (maximum correction factor ∼1.2), likely due to the relatively low optical depth and insignificant contribution from the diffuse radio continuum emission for Perseus. We apply the correction to the optically thin HI column density on a pixel-by-pixel basis and find that the total HI mass increases by ∼10%. Using the corrected HI column density image and far-infrared data from the IRIS Survey, we then derive the H₂ column density on ∼0.4 pc scales. For five dark and star-forming sub-regions, the HI surface density is uniform with Σ_HI ∼ 7-9 M_☉ pc^–2, in agreement with the minimum HI surface density required for shielding H₂ against photodissociation. As a result, Σ_H2_/ΣHI_ and Σ_HI+Σ_H2_ show a tight relation. Our results are consistent with predictions for H₂ formation in steady state and chemical equilibrium and suggest that H₂ formation is mainly responsible for the Σ_HI saturation in Perseus. We also compare the optically thick HI with the observed "CO-dark" gas and find that the optically thick HI only accounts for ∼20% of the "CO-dark" gas in Perseus.