H2 Formation in the Perseus Molecular Cloud: Observations Meet Theory
S. Stanimirovic, K. Douglas, L. Knee, J. Di Francesco, S. Gibson, A. Begum, J. Grcevich, C. Heiles, E. Korpela, A. Leroy, J. Peek, N. Pingel, M. Putman, and D. Saul
Recent studies of nearby spiral galaxies have shown an extremely tight linear relation between the surface density of star formation and the surface density of molecular gas. This suggests that stars form in molecular clouds with a constant efficiency and therefore the ability to form molecular gas controls the evolution of individual galaxies. However, what physical processes regulate the fraction of molecular gas in galaxies is not well understood. To investigate the fundamental principles of H2 formation, we focus on the HI-H2 transition across the Perseus molecular cloud on sub-pc scales (0.4 pc). We derive the HI and H2 surface density images and compare the H2-to-HI ratio of several dark and star-forming regions to the analytic model by Krumholz et al. (2009; KMT09). To derive the H2 surface density image, we use the dust column density measured by far-infrared data from IRAS in combination with HI data from the GALFA-HI survey. We find a uniform HI surface density of 6-8 solar mass pc−2 for both dark and star-forming regions, consistent with KMT09's prediction of the minimum HI column density required to shield H2 against photodissociation. As a result, we find that H2 linearly increases with the total gas column density. Our results are consistent with KMT09's model of equilibrium H2 formation and suggest that interstellar turbulence may play a secondary role in regulating the fraction of molecular gas in galaxies.