MEGADES: MEGARA galaxy disc evolution survey

Ionised gas diagnosis

¹ Departamento de Física de la Tierra y Astrofísica, Universidad Complutense de Madrid, E-28040 Madrid, Spain
² Instituto de Física de Partículas y del Cosmos IPARCOS, Facultad de Ciencias Físicas, Universidad Complutense de Madrid, E-28040 Madrid, Spain
³ Instituto Nacional de Astrofísica, Óptica y Electrónica, Luis Enrique Erro No.1, C.P. 72840 Tonantzintla, Puebla, Mexico
⁴ Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁵ Universidad Politécnica de Madrid, Madrid, Spain
⁶ FRACTAL S.L.N.E. C/ Tulipán 2, p13, 1A. E-28231 Las Rozas de Madrid, Spain

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 27 June 2025
Accepted: 17 October 2025

Abstract

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. The technical capabilities of MEGARA (Multi-Espectrógrafo en GTC de Alta Resolución para Astronomía) provide us with relatively high spectral (R ∼ 6000) and spatial (0.62″) resolution observations that we used to study the properties of the ionised gas via various methods, including using the classic diagnostic BPT diagrams of its [N II] and [S II] variants. We explore how the diagrams vary as a function of both the radius and velocity dispersion of the Hα line. We also propose a new diagnostic diagram for assessing the relative contributions of active galactic nuclei (AGN), shocks, and H II regions in each spatial region as the ratio between the velocity dispersion of the [N II]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 [N II]λ6584/Hα and intermediate [O III]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 allowed for a robust oxygen abundance determination using the N2 indicator, which we used to measure precise abundance gradients. Most galaxies show negligible metallicity gradients, especially the low-abundance (< 8.37 dex) fast rotators. The mean value of the slope of the metallicity gradients for this subset is 0.005 dex R_e⁻¹, with a dispersion of 0.422 dex R_e⁻¹. Above 8.37 dex the fast rotators consistently show slightly negative metallicity gradients, with a weak correlation between the slope and the y-intercept. The mean slope of these galaxies is −0.681 dex R_e⁻¹, with a dispersion of 0.933 dex R_e⁻¹. The overall mean value of the gradients for all galaxies in the MEGADES sample is −0.025 dex R_e⁻¹, with a dispersion of 0.766 dex R_e⁻¹. We discuss the possible implications of these results regarding the impact of galactic winds on the abundance gradients of galaxies.