Dense gas relation of N₂H⁺ vs. HCN as shown in Fig. 2, but color-coded by HCN/HCO⁺. This ratio is an alternative diagnostic for AGN contamination of molecular lines, with a ratio greater than 1.8 usually corresponding to obscured nuclear activity (Kohno 2005). Yet the main correlation between N₂H⁺ (dominated by star-forming pixels) has many pixels with HCN/HCO⁺ > 1.8. Indeed, the pixels with the largest HCN/HCO⁺ ratio are not those with the greatest difference from the average trend, but those with a moderate difference but low N₂H⁺ emission. Pixels within the central 0.5 kpc have a dark border, to demonstrate how this ratio decreases far more gradually than the ELR function moving away from the galaxy center.