Observational Evidence of Different Evolutionary Stages in Galactic Bars

Abstract

We analyze the stellar line-strength index distribution along the bar of a sample of 20 early-type galaxies derived from optical long-slit observations along the bar major axis. The aim is to study the formation and evolution of bars in galaxies. We obtain age and metallicity distributions using stellar population models. We find that the mean bar values of age, metallicity and [E/Fe] correlate with central velocity dispersion in a similar way to that of bulges, pointing to a intimate evolution of both components. Galaxies with high stellar velocity dispersions (>170kms−1) host bars with old stars while galaxies with lower central velocity dispersion show stars with a large dispersion in their ages. We find, for the first time, gradients in both age and metallicity. We find three different types of bars according to their metallicity and age distribution along the radius: (1) bars with negative metallicity gradients. They show mean young/intermediate population (<2Gyr), and have amongst the lowest stellar velocity dispersion of the sample. (2) Bars with null metallicity gradients. The galaxies that do not show any gradient in their metallicity distribution along the bar and have positive age gradients. (3) Bars with a mean older population and positive metallicity gradients, i.e. more metal rich at the bar ends. The results indicate that most bars are long-lasting structure. These derived gradients place strong constrains on models of bar evolution. All the galaxies show disk-like central components, implying a strong role played by bars in the bulge secular evolution.