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Evolution of Giant Molecular Clouds in Nearby Galaxies

Studies of GMC evolution in galactic disks were limited to local, predominantly atom-rich small galaxies in the past, but have now been expanded to typical spiral galaxies with a rich molecular content. The evolution appears quite different between the two environments. GMCs exist almost exclusively along HI spiral arms and filaments in the disks of local small galaxies (LMC, M33), suggesting that GMCs form and end their short lives there. However, in a more molecular-rich environment (MW, M51), GMCs are present everywhere independent of HI structures. Indeed, the molecular gas fraction remains high and almost constant during arm passage into the next inter-arm region. The gas remains molecular, presumably in GMCs, for a long time. A transitional case has been found recently in the central region of the atom-rich galaxy M33 - GMCs do not coincide with HI there. Evolution of the physical conditions of molecular gas from spiral arms to inter-arm regions is also being revealed in molecule-rich galaxies. An increase of the CO J=2-1 and 1-0 line ratio in spiral arms in M51 suggests density and/or temperature increases by a factor of 2-3 in GMCs in the arms, compared to their counterparts in the inter-arm regions. An analysis of high-resolution Milky Way survey data revealed that the fraction of dense (or warm) clumps increases dramatically in the spiral arms.