Study of stellar populations in the bulges of barred galaxies

Abstract

We have obtained long-slit spectroscopy for a sample of 20 early-type barred galaxies to study the influence of bars in the building of galaxy bulges. Line strength indices were measured and used to derive age and metallicity gradients in the bulge region by comparing with stellar population models. The same analysis was also carried out with similar data of unbarred galaxies taken from the literature. The bulges of barred galaxies seem to be more metal rich, at a given velocity dispersion (sigma), than the bulges of unbarred galaxies, as measured by some metallicity sensitive indices. There are indications that the ratio of relative abundance of alpha-elements with respect to iron, [E/Fe], derived for the bulges of barred galaxies tend to lie above the values of the unbarred galaxies at a given sigma. The metallicity gradients for the majority of the bulges are negative, less metal rich towards the end of the bulge. The gradient values show a large scatter for galaxies with sigma below 150 km/s. The age distribution is related to the presence of bulge substructure such as a nuclear ring or an inner disk. The metallicity of both the bulge and the bar are very well correlated indicating a close link between the enrichment histories of both components. Bulges of barred early type galaxies might have suffered a different chemical enrichment compared to the bulges of unbarred galaxies of the same morphological type, same central velocity dispersion and low inclination angles. The hinted stellar populations differences separating the bulges of barred and unbarred galaxies and the strong link existing between the metallicity of the bulge and the presence of a bar points to scenarios were they both form simultaneously in processes leading to rapid and massive episodes of star formation, possibly linked to the bar formation.