MEGADES: MEGARA galaxy disc evolution survey. Ionised gas diagnosis

We present the ionised gas properties and metallicity gradients of 43 galaxies observed by the MEGADES survey. Using the MEGARA (Multi-Espectrografo en GTC de Alta Resolucion para Astronomia) instrument, our data combine relatively high spectral (R ~ 6000) and spatial (0.62 arcsec) resolution to study the ionised gas through classic BPT diagnostics in the [N II] and [S II] variants. We examine how these diagrams vary with radius and with the velocity dispersion of the H-alpha line, and we propose a new diagnostic based on the ratio between the velocity dispersions of the [N II] 6584 and H-alpha lines to disentangle the relative roles of AGN, shocks and H II regions. Many regions, regardless of galactocentric distance, show shock-like emission, inferred from their line ratios (high [N II]6584/H-alpha and intermediate [O III]5007/H-beta) and their location between H II and AGN regimes. This improved selection of H II-like regions enables robust oxygen abundance estimates using the N2 indicator. Most galaxies show negligible metallicity gradients, especially low-abundance (<8.37 dex) fast rotators, with an average slope of 0.005 dex Re^-1 and a dispersion of 0.42 dex Re^-1. Above 8.37 dex, fast rotators display mildly negative gradients (mean -0.68 dex Re^-1, dispersion 0.93). For the full MEGADES sample, the mean gradient is -0.03 dex Re^-1 with a dispersion of 0.77. We discuss the implications for the influence of galactic winds on abundance gradients.