Star formation history of barred disc galaxies

Abstract

We present the first results of a pilot study aimed at understanding the\ninfluence of bars on the evolution of galaxy discs through the study of their\nstellar content. We examine here the kinematics, star formation history,\nmass-weighted, luminosity-weighted, and single stellar population (SSP)\nequivalent ages and metallicities for four galaxies ranging from lenticulars to\nlate-type spirals. The data employed extends to 2-3 disc scalelengths, with\nS/N(A)>50. Several techniques are explored to derive star formation histories\nand SSP-equivalent parameters, each of which are shown to be compatible. We\ndemostrate that the age-metallicity degeneracy is highly reduced by using\nspectral fitting techniques --instead of indices-- to derive these parameters.\nWe found that the majority of the stellar mass in our sample is composed of old\n(~10 Gyr) stars. This is true in the bulge and the disc region, even beyond two\ndisc scalelengths. In the bulge region, we find that the young, dynamically\ncold, structures produced by the presence of the bar (e.g., nuclear discs or\nrings) are responsible for shaping the bulges' age and metallicity gradients.\nIn the disc region, a larger fraction of young stars is present in the external\nparts of the disc compared with the inner disc. The disc growth is, therefore,\ncompatible with a moderate inside-out formation scenario, where the luminosity\nweighted age changes from ~10 Gyrs in the centre, to ~4 Gyrs at two disc\nscalelengths, depending upon the galaxy. For two galaxies, we compare the\nmetallicity and age gradients of the disc major axis with that of the bar,\nfinding very important differences. In particular, the stellar population of\nthe bar is more similar to the bulge than to the disc, indicating that, at\nleast in those two galaxies, bars formed long ago and have survived to the\npresent day. (abridged)\n