A High-Resolution Study of the H I-H₂transition Across the Perseus Molecular Cloud

To investigate the fundamental principles of H 2 formation in a giant molecular cloud, we derive the H i and H 2 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 ☉ pc −2 for both dark and star-forming regions, suggesting a minimum H i surface density required to shield H 2 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 H 2 -dominated regions occurs at N (H i ) + 2 N (H 2 ) ∼ (8–14) × 10 20 cm −2 . Our findings are consistent with predictions for H 2 formation in equilibrium, suggesting that turbulence may not be of primary importance for H 2 formation. However, the importance of a warm neutral medium for H 2 shielding, an internal radiation field, and the timescale of H 2 formation still remain as open questions. We also compare H 2 and 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.