Spectral Line Survey toward Spiral Arm of M51 in the 3 mm and 2 mm Bands
  We have conducted a spectral line survey in the 3 mm and 2 mm bands toward
two positions in a spiral arm of M51 (NGC 5194) with the IRAM 30 m telescope.
In this survey, we have identified 13 molecular species, including CN, CCH,
N2H+, HNCO, and CH3OH. Furthermore, 6 isotopologues of the major species have
been detected. On the other hand, SiO, HC3N, CH3CN, and the deuterated species
such as DCN and DCO+ are not detected. The deuterium fractionation ratios are
evaluated to be less than 0.8 % and 1.2 % for DCN/HCN and DCO+/HCO+,
respectively. By comparing the results of the two positions with different star
formation activities, we have found that the observed chemical compositions do
not strongly depend on star formation activities. They seem to reflect a
chemical composition averaged over the 1-kpc scale region including many giant
molecular clouds. Among the detected molecules CN, CCH, and CH3OH are found to
be abundant. High abundances of CN, and CCH are consistent with the above
picture of a wide spread distribution of molecules, because they can be
produced by photodissociation. On the other hand, it seems likely that CH3OH is
liberated into the gas phase by shocks associated with large scale phenomena
such as cloud-cloud collisions and/or by non-thermal desorption processes such
as photoevaporation due to cosmic-ray induced UV photons. The present result
demonstrates a characteristic chemical composition of a giant molecular cloud
complex in the spiral arm, which can be used as a standard reference for
studying chemistry in AGNs and starbursts.
