Map of the ELR [NII] function over the SWAN field of view of M51, with HCN emission line contours shown in black for reference. The BPT diagram used to establish the ELR is shown in the top right, with values ranging from 0–0.25 for star-forming regions as defined by the Kauffmann et al. (2003) criteria (dashed green line) and values exceeding 0.5 for AGN regions (above the Kewley et al. (2001) criteria, orange line). The central 0.5 kpc is shown as a dashed pink line, to demonstrate how simple radial cuts to exclude AGN activity leave out the internal complexity of ionizing sources.