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Central enhancement of the nitrogen-to-oxygen abundance ratio in barred galaxies

Bar-induced gas inflows towards the galaxy centres are recognized as a key agent for the secular evolution of galaxies. One immediate consequence is the accumulation of gas in the centre of galaxies where it can form stars and alter the chemical and physical properties. We use a sample of nearby face--on disc galaxies with available SDSS spectra to study whether the properties of the ionised gas in the central parts (radii <~0.6-2.1 kpc) of barred galaxies are altered by the presence of a bar, and whether the bar effect is related to bar and/or parent galaxy properties. The distributions of all parameters analysed are different for barred and unbarred galaxies, except for the R23 metallicity tracer and the oxygen abundance (from photoionisation models). The median values point towards (marginally) larger dust content, star formation rate per unit area, electron density and ionisation parameter in the centres of barred galaxies than in the unbarred counterpart. The most remarkable barred/unbarred difference appears in the [NII]6583/Ha line ratio, which is on average ~25% larger in barred galaxies, due to a larger N/O in the centres of these galaxies. We observe an enhancement of the central gas differences in later-type galaxies or galaxies with less massive bulges. However the bar seems to have a lower impact on the central gas properties for galaxies with more massive bulges (M_bulge > 10^10 M_sun) or galaxies with total stellar mass above ~ 10^10.8 M_sun. In conclusion, we find observational evidence that the presence of a galactic bar affects the central ionised gas properties of disc galaxies, where the most striking effect is an enhancement in the N/O abundance ratio, which can be qualitatively interpreted as due to a different origin or evolutionary processes for less and more massive bulges, with the gaseous phase of the former having currently a closer relation with bars.