Title: Evolution of Molecular Gas and Star Formation in Galaxies

Abstract: I will discuss the large-scale distribution, evolution, and dynamics of giant molecular clouds (GMCs) in galaxies. The talk is mostly based on the results on M51, but I will also summarize recent results from the CARMA and NObeyama Nearby-galaxies (CANON) CO 1-0 survey and the CO observations of the Milky Way. The standard, albeit simplistic, picture of ISM phases posits that GMCs are assembled in the spiral arm shocks from diffuse interarm HI gas and then photo-dissociated back into the atomic phase by OB star formation within the spiral arms. However, we are finding many GMCs both on spiral arms and in interarm regions, indicating their long lifetime comparable to galactic rotation timescale. The associations of GMCs (so-called GMAs) are found only on spiral arms, and thus, they are likely unbound, short-lived structures, being broken up across spiral arms. A molecular gas fraction stays high even in interarm regions. Therefore, the GMA destruction is not likely caused by stellar feedback such as strong UV radiation or supernovae, since they would destroy molecules as well as GMAs and GMCs. Instead, I will discuss a picture of dynamically-driven evolution -- strong shear motions in spiral arms cause the GMA destruction and trigger GMC evolution. At small scales, dense gas cores are expected to develop in GMCs through spiral arms, leading to star formation. I will also discuss evidence for such growth in nearby spiral galaxies and in the Milky Way.