MEGADES: MEGARA galaxy disc evolution survey. Ionised gas diagnosis

TL;DR

This study uses high-resolution spectroscopic data from the MEGADES survey to analyze ionized gas properties and metallicity gradients in 43 galaxies, introducing a new diagnostic to distinguish ionization sources and examining their impact on galaxy evolution.

Contribution

The paper presents a novel diagnostic based on velocity dispersion ratios to differentiate ionization mechanisms and provides detailed metallicity gradient measurements across a diverse galaxy sample.

Findings

Many regions show shock-like emission regardless of galactocentric distance.

Most galaxies have negligible metallicity gradients, especially low-abundance fast rotators.

Fast rotators with higher metallicity display mildly negative gradients.

Abstract

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.