Molecular Distribution in the Spiral Arm of M51
  Molecular line images of 13CO, C18O, CN, CS, CH3OH, and HNCO are obtained
toward the spiral arm of M51 at a 7" times 6" resolution with the Combined
Array for Research in Millimeter-wave Astronomy (CARMA). Distributions of the
molecules averaged over a 300 pc scale are found to be almost similar to one
another and to essentially trace the spiral arm. However, the principal
component analysis shows a slight difference of distributions among molecular
species particularly for CH3OH and HNCO. These two species do not correlate
well with star-formation rate, implying that they are not enhanced by local
star-formation activities but by galactic-scale phenomena such as spiral
shocks. Furthermore, the distribution of HNCO and CH3OH are found to be
slightly different, whose origin deserves further investigation. The present
results provide us with an important clue to understanding the 300 pc scale
chemical composition in the spiral arm and its relation to galactic-scale
dynamics.
