Integrated intensity maps of N₂H⁺ (top left) and HCN (top center), as well as their ratio (top right) at 3″ (∼125 pc) resolution of the central 5 kpc  ×  7 kpc in M 51a. The ratio map shows emission above 3σ for both lines. The beam of ∼3″ is shown in the bottom left corner of the N₂H⁺ map for reference; the location of the galactic center is marked (green ×). We further display ¹²CO emission at 3″ resolution from the PAWS survey (bottom left; Schinnerer et al. 2013) for comparison and show the 30 K km s⁻¹ contour of ¹²CO for reference in all maps. The central 1.5 kpc (in diameter) is indicated by a cyan circle in the ¹²CO map. Average spectra of five beam-sized regions in the disk (see the ¹²CO map) are shown for N₂H⁺, HCN and ¹²CO (bottom center). We scale the spectra by a factor of 3 (N₂H⁺) and 0.05 (¹²CO) for easier comparison. The full-disk spectra contain all pixels in the FoV, shown on top of a HST image (bottom right).