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Paper: The Influence of Shear Motion on Evolution of Giant Molecular Associations
Volume: 480, Structure and Dynamics of Disk Galaxies
Page: 259
Authors: Miyamoto, Y.; Nakai, N.; Kuno, N.
Abstract: The evolution of Giant Molecular Clouds (GMCs) and Giant Molecular Associations (GMAs) is one of the keys to understand massive star formation in a galaxy and hence evolution of the galaxy. It has been considered that GMCs and GMAs are formed in spiral arms. Recently, however, GMCs are found not only in the spiral arms but also in the inter-arms. It is suggested that the GMCs in the inter-arms might be formed by the shear motion. However, the relation between molecular clouds and the kinetic shear motion in the clouds is still speculation, because the kinetic shear has not been directly measured in a galaxy. We have investigated the dynamics of the molecular gas and the evolution of GMAs in the spiral galaxy M51 with the NRO 45-m telescope. The velocity components of the molecular gas perpendicular and parallel to the spiral arms were derived at each spiral phase from the distribution of the line-of-sight velocity of the CO gas. The shear motion in the galactic disk was determined from the velocity vectors at each spiral phase. It was revealed that the distributions of the shear strength and of GMAs are anti-correlated. In addition, GMAs can grow up just in regions where the gravitational critical density is larger than the critical shear density. This result suggests that the evolution of GMAs is heavily affected by the shear.
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