Study of the stellar line-strength indices and kinematics along bars⋆ I. Bar age and metallicity gradients

Aims. This is the first paper of a series aimed to understand the formation and evolution of bars in early-type spirals and their influence in the evolution of the galaxy. Methods. Optical long-slit spectra along the major axis of the bar of a sample of 20 galaxies are analyzed. velocity and velocity dispersion profiles along the bar are presented. Line-strength indices in the bar region are also measured to derive stellar meanage and metallicity distributions along the bars using stellar population models. Results. We obtain mean ages, metallicities and chemical abundances along the bar of 20 galaxies with morphological types from SB0 to SBbc. The main result is that we find large variation in age and metallicity along the bar in 45% of our sample. 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 (< 2 Gyr), and have amongst the lowest stellar maximum central velocity dispersion of the sample. 2) Bars with null metallicity gradients. These galaxies that do not show any gradient in their metallicity distribution along the bar and have negative age gradients (i.e younger populations at the bar end). 3) Bars with positive metallicity gradients, i.e. more metal rich at the bar ends. These galaxies are predominantly those with higher velocity dispersion and older mean population. We found no significant correlation between the age and metallicity distribution, and bar/galaxy parameters such as the AGN presence, size or the bar strength. From the kinematics, we find that all the galaxies show a disk–like central component. Conclusions. The results from the metallicity and age gradients indicate that most galaxies with high central stellar velocity dispersion host bars that could have been formed more than 3 Gyrs ago, while galaxies with lower central velocity dispersions show a wider distribution in their population and age gradients. A few bars show characteristics compatible with having been formed less than <2 Gy ago. However, we do not have a definite answer to explain the observed gradients and these results place strong constrains to models of bar formation and evolution. The distribution of mean stellar population parameters in the bar with respect to σ is similar to that found in bulges, indicating a close link in the evolution of both components. The disk-like central components also show the important role played by bars in the secular evolution of the central structure.