Jin Koda

Stony Brook University

Evolution of Giant Molecular Clouds in Galaxies

I will discuss the large-scale distribution, evolution, and dynamics of giant molecular clouds (GMCs) in galaxies. The discussion will be based primarily on recent CO(1-0) observations of M51 with the CARMA interferometer and Nobeyama 45m telescope, and on the recent Galactic plane 13CO survey. In particular, we find many GMCs both on spiral arms and in interarm regions in M51, indicating they have a long lifetime comparable to the galactic rotation timescale. Associations of giant molecular clouds (so-called GMAs) are found only on spiral arms, and thus, they are likely unbound, short-lived structures, being broken up across spiral arms. The molecular gas fraction is high even in interarm regions; therefore, GMA destruction is not likely caused by stellar feedback such as strong UV radiation or supernovae, since these processes would destroy molecules as well as GMAs and GMCs. Instead, I will discuss dynamically-driven GMC evolution, in which strong shear motions in spiral arms cause GMA destruction and trigger GMC evolution.