2012ApJ...748...75L

To investigate the fundamental principles of H₂ formation in a giant molecular cloud, we derive the H I and H₂ surface density (Σ_H I _ and Σ_H2) images of the Perseus molecular cloud on sub-pc scales (∼0.4 pc). We use the far-infrared data from the Improved Reprocessing of the IRAS Survey and the V-band extinction image provided by the COMPLETE Survey to estimate the dust column density image of Perseus. In combination with the H I data from the Galactic Arecibo L-band Feed Array H I Survey and an estimate of the local dust-to-gas ratio, we then derive the Σ_H2 distribution across Perseus. We find a relatively uniform Σ_ H I_∼ 6-8 M_☉/pc2 for both dark and star-forming regions, suggesting a minimum H I surface density required to shield H₂ against photodissociation. As a result, a remarkably tight and consistent relation is found between Σ_H2/Σ_ H I_and Σ_H I_+ Σ_H2. The transition between the H I- and H₂-dominated regions occurs at N(H I) + 2N(H₂) ∼ (8-14)x10²⁰/cm2. Our findings are consistent with predictions for H₂ formation in equilibrium, suggesting that turbulence may not be of primary importance for H₂formation. However, the importance of a warm neutral medium for H₂shielding, an internal radiation field, and the timescale of H₂ formation still remain as open questions. We also compare H₂and CO distributions and estimate the fraction of "CO-dark" gas, f_DG∼ 0.3. While significant spatial variations of f_DG are found, we do not find a clear correlation with the mean V-band extinction.