2011MNRAS.415..709S

We present the first results of a pilot study aimed at understanding the influence of bars on the evolution of galaxy discs through the study of their stellar content. We examine here the kinematics, star formation history, mass-weighted, luminosity-weighted and single stellar population (SSP) equivalent ages and metallicities for four galaxies ranging from lenticulars to late-type spirals. The data employed extend to 2–3 disc scalelengths, with S/N(Å) > 50, where S/N stands for the signal-to-noise ratio. Several techniques are explored to derive star formation histories and SSP-equivalent parameters, each of which is shown to be compatible. We demonstrate that the age–metallicity degeneracy is highly reduced by using spectral fitting techniques – instead of indices – to derive these parameters. Our results are robust to the choices of stellar population models. We found that the majority of the stellar mass in our sample is composed of old (∼10 Gyr) stars. This is true in the bulge and disc regions, even beyond 2 disc scalelengths. In the bulge region, we find that the young, dynamically cold, structures produced by the presence of the bar (e.g. nuclear discs or rings) are responsible for shaping the bulges' age and metallicity gradients, as suggested by Peletier et al. In the disc region, a larger fraction of young stars are present in the external parts of the disc compared with the inner parts. The disc growth is therefore compatible with a moderate inside-out formation scenario, where the luminosity-weighted age changes from ∼10 Gyr in the centre to ∼4 Gyr at 2 disc scalelengths, depending upon the galaxy. However, the presence of substructures, like star-forming rings, can produce stellar population trends that are not directly related to the growing of the disc but to the bar potential. For two galaxies, we compare the metallicity and age gradients of the disc major-axis with that of the bar, finding very important differences. In particular, the stellar population of the bar is more similar to that of the bulge than to that of the disc, indicating that, at least in those two galaxies, bars formed long ago and have survived to the present day.