Diagnostic plots using integrated line ratios of HCN, HNC and HCO⁺ versus each other. Left panels: (a): integrated HCO⁺/HCN versus HNC/HCO⁺ ratios. (b): integrated HCO⁺/HCN versus HNC/HCN ratios. (c): integrated HNC/HCN versus HNC/HCO⁺ ratios. The grey dashed lines marks the boarder between XDR and PDR (Baan et al. 2008; Loenen et al. 2008). Circles indicate reliable values (S/N > 5) and upper limits for the central 20″ ≈ 745 pc towards NGC 6946. Stars show the EMPIRE galaxies and stars with red contours present galaxy centres hosting an AGN (Goulding & Alexander 2009). For two of them we find ancillary data: the cross shaped markers show the ∼4″ (≈166 pc) M 51 observations (Querejeta et al. 2016) and the ∼2″ (≈100 pc) NGC 3627 observations (Bešlić et al. 2021). (d): the enclosed integrated HNC/HCN versus HNC/HCO⁺ with the kpc-sized centres of the EMPIRE survey (see Jiménez-Donaire et al. 2019 for details). NGC 6946 and the EMPIRE galaxies are in the PDR regime in all panels. Interestingly, EMPIRE galaxies harbouring an AGN do not show an enhancement of HCN as suggested in many studies (see text for a discussion). We run (Meijerink & Spaans 2005) PDR models for the observed ranges of line ratios (blue and red shaded areas) by (i) fixing the radiation field and (ii) fixing the density. The red and blue lines show the model output (more in text). The colours in (a–d) indicate Σ_SFR. We have normalised them to the mean Σ_SFR value and find no trend with the integrated line ratios. Upper right panel: X-ray 0.5 − 7.0 keV in counts for NGC 6946 with integrated CO(2−1) contours on top. The black crosses mark the positions of the detected X-ray sources by Holt et al. (2003) which are not strong enough to trigger a XDR.