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We examine the relation between polycyclic aromatic hydrocarbon (PAH)emission at 8 microns and far-infrared emission from hot dust grains at 24microns and from large dust grains at 160 and 250 microns in the nearby spiralgalaxies NGC 2403 and M83 using data from the Spitzer Space Telescope andHerschel Space Observatory. We find that the PAH emission in NGC 2403 is bettercorrelated with emission at 250 microns from dust heated by the diffuseinterstellar radiation field (ISRF) and that the 8/250 micron surfacebrightness ratio is well-correlated with the stellar surface brightness asmeasured at 3.6 microns. This implies that the PAHs in NGC 2403 are intermixedwith cold large dust grains in the diffuse interstellar medium (ISM) and thatthe PAHs are excited by the diffuse ISRF. In M83, the PAH emission appears morestrongly correlated with 160 micron emission originating from large dust grainsheated by star forming regions. However, the PAH emission in M83 is low wherethe 24 micron emission peaks within star forming regions, and enhancements inthe 8/160 micron surface brightness ratios appear offset relative to the dustand the star forming regions within the spiral arms. This suggests that thePAHs observed in the 8 micron band are not excited locally within star formingregions but either by light escaping non-axisymmetrically from star formingregions or locally by young, non-photoionising stars that have migrateddownstream from the spiral density waves. The results from just these twogalaxies show that PAHs may be excited by different stellar populations indifferent spiral galaxies.

The relationship between polycyclic aromatic hydrocarbon emission and far-infrared dust emission from NGC 2403 and M83