Influence of Shear Motion on Evolution of Molecular Clouds in the Spiral
  Galaxy M51
  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
are derived at each spiral phase from the distribution of the line-of-sight
velocity of the CO gas. In addition, the shear motion in the galactic disk is
determined from the velocity vectors at each spiral phase. It is revealed that
the distributions of the shear strength and of GMAs are anti-correlated. GMAs
exist only in the area of the weak shear strength and further on the upstream
side of the high shear strength. GMAs and most of GMCs exist in the regions
where the shear critical surface density is smaller than the gravitational
critical surface density, indicating that they can stably grow by self-gravity
and the collisional agglomeration of small clouds without being destroyed by
shear motion. These indicate that the shear motion is an important factor in
evolution of GMCs and GMAs.
