Molecular Cloud Evolution as the first step for Star Formation and Galaxy Evolution
Abstract
Molecular gas and clouds host virtually all star formation, and therefore, their formation and evolution are the first step leading to star formation and galaxy evolution. On the basis of broad observational data, I will discuss how molecular gas and clouds evolve in the Milky Way and nearby spiral galaxies. In particular, I will argue for a long cloud lifetime (>~100 Myr), as opposed to the recently-suggested short lifetime (<~10-30 Myr). Simply put, we see molecular gas but don’t see much atomic gas within galactic disks, even in the inter-arm regions, and thus the gas stays molecular. In this picture, star formation does not occur at the onset of gravitational collapse from atomic gas to molecular clouds, but is triggered in the long-existing molecular clouds. This view contradicts the traditional picture of the spiral density-wave theory, which predicts a rapid gas phase transition — from atomic to molecular and then to atomic — through spiral arm passage. Instead, the gas evolves through the coagulation and fragmentation of molecular clouds. The consequences of this revised view of cloud evolution, star formation and galaxy evolution will be outlined.