MEGADES: MEGARA galaxy disc evolution survey. Ionised gas diagnosis.

We present the ionised gas properties and metallicity gradients of the central area of a sample of 43 galaxies using observations obtained by the MEGADES survey. Exploiting the technical capabilities of MEGARA (Multi-Espectrógrafo en GTC de Alta Resoluciónpara Astronomía) our observations combine relatively high spectral (R ∼ 6000) and spatial (0.62") resolution to study the properties of the ionised gas, such as using the classic diagnostic BPT diagrams in its [NII] and [SII] variants. We explore how the diagrams vary as a function of both radius and velocity dispersion of the Hα line. We also propose a new diagnostic diagram to assess the relative contributions of AGN, shocks and H II regions in each spatial region as the ratio between the velocity dispersion of the [NII]λ6584 and Hα lines. A considerable number of regions, regardless of their galactocentric distance, have emission line spectra associated with shocks. This inference follows both from their line ratios, typically characterised by high [NII]λ6584/Hα and intermediate [OIII]λ5007/Hβ, and from their position in our diagnostic diagram where they lie between the areas associated with HII regions and with AGN. The better selection of HII-region like emission allows for a robust oxygen abundance determination using the N2 indicator, which we use to measure precise abundance gradients. We discuss the possible implications of these results for the impact of galactic winds on the abundance gradients of galaxies.