title: Chemical Compositions and Distributions in Arm of M51

authors: Yoshimasa Watanabe, Nami Sakai (Univ. Tokyo), Kazuo Sorai (Hokkaido Univ.), Satoshi Yamamoto (Univ. Tokyo)

abstract: Recently, chemistry of external galaxies has attracted more attention of astronomers than before, because various molecular species become readily detected in nearby galaxies.  In general, chemical compositions observed toward external galaxies are 'average' chemical compositions of a number of giant molecular clouds (GMCs).  If the physical meaning of the GMC-scale chemical compositions is adequately established, we can make use of it as a new tracer to diagnose external galaxies.  With this motivation, we have carried out the spectral line survey toward the spiral arm of M51 with the IRAM 30 m telescope and detected 13 molecular species (Watanabe et al. 2014).  The excitation temperatures of the molecules are estimated to be less than 10 K.  Therefore, the molecules would reside in a cold and widespread molecular gas, although a part of molecular may also reside in the warm molecular gas around star forming regions.  Based on the spectral line survey, we have conducted mapping observation of 13CO, C18O, CN, CH3OH, CS, and HNCO with the CARMA.  Distributions of other species are found to be different from molecule to molecule.  These differences would originate from the environmental difference among GMCs and/or the dynamics in the spiral arm such as cloud-cloud collisions.