2016ApJ...819..144W

Molecular line images of ¹³CO, C¹⁸O, CN, CS, CH₃OH, and HNCO are obtained toward the spiral arm of M51 at a 7^′′ ×6^′′ resolution with the Combined Array for Research in Millimeter-wave Astronomy. 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 the 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 distributions of HNCO and CH₃OH are found to be slightly different, and their origins deserve further investigation. The present results provide us with an important clue for understanding the 300 pc scale chemical composition in the spiral arm and its relation to galactic-scale dynamics.