Ionised gas abundances in barred spiral galaxies

Aims. This is the third paper of a series devoted to studying the properties of bars from long-slit spectroscopy to understand their formation, evolution, and influence on the evolution of disk galaxies. In this paper, we aim to determine the gas metallicity distribution of a sample of 20 barred early-type galaxies. We compare the nebular and stellar metallicity distributions to attempt to infer the origin of the warm gas. Methods. We performed long-slit spectroscopy along the bar and obtained metallicities derived using different calibrations. We compare the nebular emission metallicities derived using different semi-empirical methods. We carry out AGN diagnostic diagrams for data at different radii to determine the radius of influence of the AGN and the nature of the galactic nuclei. We then derive the gas metallicities along the bars and compare our results to the distribution of stellar metallicities in the same regions. Results. Most of the gas emission is centrally concentrated, although 15 galaxies also show emission along the bar. In the central regions, gas oxygen abundances are in the range 12 + log (O/H) = 8.4−9.1. The nebular metallicity gradients are very shallow in the bulge and bar regions. For three galaxies (one of them a low-ionization nuclear emission-line region), the gas metallicities lie well below the stellar ones in the bulge region. These results do not depend on the choice of semi-empirical calibration used to calculate the abundances. We see that the galaxies with the lowest metallicities are those with the largest rotational velocities. Unlike our stellar metallicities, we find no correlation between the nebular abundances and the central velocity dispersion. In most galaxies, the gradient in the gas nebular metallicities in the bulge region is shallower than that for the stellar metallicity. Conclusions. The presence of gas of significantly lower metallicity than the stellar abundances in three of our galaxies, suggests that the gas has an external origin that fuels the present star formation at the center of some early-type barred galaxies. That the bar/disk nebular metallicities are higher than the central ones might indicate that the gas could be accreted via cooling flows instead of radial accretion from gas sitting in the outer parts of the disk