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We present the first complete CO J=3-2 map of the nearby grand-design spiralgalaxy M51 (NGC 5194), at a spatial resolution of ~600 pc, obtained with theHARP-B instrument on the James Clerk Maxwell Telescope. The map covers theentire optical galaxy disk and out to the companion NGC 5195, with CO J=3-2emission detected over an area of ~9'x6' (~21x14 kpc). We describe the CO J=3-2integrated intensity map and combine our results with maps of CO J=2-1, COJ=1-0 and other data from the literature to investigate the variation of themolecular gas, atomic gas and polycyclic aromatic hydrocarbon (PAH) propertiesof M51 as a function of distance along the spiral structure on sub-kpc scales.We find that for the CO J=3-2 and CO J=2-1 transitions there is a cleardifference between the variation of arm and inter-arm emission withgalactocentric radius, with the inter-arm emission relatively constant withradius and the contrast between arm and inter-arm emission decreasing withradius. For CO J=1-0 and HI the variation with radius shows a similar trend forthe arm and inter-arm regions, and the arm-inter-arm contrast appearsrelatively constant with radius. We investigate the variation of CO line ratios(J=3-2/2-1, J=2-1/1-0 and J=3-2/1-0) as a function of distance along the spiralstructure. Line ratios are consistent with the range of typical values forother nearby galaxies in the literature. The highest CO J=3-2/2-1 line ratiosare found in the central ~1 kpc and in the spiral arms and the lowest lineratios in the inter-arm regions.We find no clear evidence of a trend withradius for the spiral arms but for the inter-arm regions there appears to be atrend for all CO line ratios to increase with radius. We find a strongrelationship between the ratio of CO J=3-2 intensity to stellarcontinuum-subtracted 8mu PAH surface brightness and the CO J=3-2 intensity thatappears to vary with radius.

A CO J=3-2 map of M51 with HARP-B: Radial properties of the spiral structure