A CO J=3-2 map of M51 with HARP-B: Radial properties of the spiral
  structure
  We present the first complete CO J=3-2 map of the nearby grand-design spiral
galaxy M51 (NGC 5194), at a spatial resolution of ~600 pc, obtained with the
HARP-B instrument on the James Clerk Maxwell Telescope. The map covers the
entire optical galaxy disk and out to the companion NGC 5195, with CO J=3-2
emission detected over an area of ~9'x6' (~21x14 kpc). We describe the CO J=3-2
integrated intensity map and combine our results with maps of CO J=2-1, CO
J=1-0 and other data from the literature to investigate the variation of the
molecular gas, atomic gas and polycyclic aromatic hydrocarbon (PAH) properties
of 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 clear
difference between the variation of arm and inter-arm emission with
galactocentric radius, with the inter-arm emission relatively constant with
radius and the contrast between arm and inter-arm emission decreasing with
radius. For CO J=1-0 and HI the variation with radius shows a similar trend for
the arm and inter-arm regions, and the arm-inter-arm contrast appears
relatively 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 spiral
structure. Line ratios are consistent with the range of typical values for
other nearby galaxies in the literature. The highest CO J=3-2/2-1 line ratios
are found in the central ~1 kpc and in the spiral arms and the lowest line
ratios in the inter-arm regions.We find no clear evidence of a trend with
radius for the spiral arms but for the inter-arm regions there appears to be a
trend for all CO line ratios to increase with radius. We find a strong
relationship between the ratio of CO J=3-2 intensity to stellar
continuum-subtracted 8mu PAH surface brightness and the CO J=3-2 intensity that
appears to vary with radius.
