| Abstract | 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 (0.4pc). 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 I6-8 Mpc -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) + 2N(H 2) (8-14)× 10 20cm -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. © 2012. The American Astronomical Society. All rights reserved.. |
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