Study of the stellar population properties in the discs of ten spiral galaxies

Abstract

We investigated the properties of the stellar populations in the discs of a sample of 10 spiral galaxies. Our analysis focused on the galaxy region where the disc contributes more than 95 per cent of total surface brightness in order to minimize the contamination of the bulge and bar. The luminosity-weighted age and metallicity were obtained by fitting the galaxy spectra with a linear combination of stellar population synthesis models, while the total overabundance of α-elements over iron was derived by measuring the line-strength indices. Most of the sample discs display a bimodal age distribution and they are characterized by a total [α/Fe] enhancement ranging from solar and supersolar.We interpreted the age bimodality as due to the simultaneous presence of both a young (Age≤4 Gyr) and an old (Age>4 Gyr) stellar population. The old stellar component usually dominates the disc surface brightness and its light contribution is almost constant within the observed radial range. For this reason, no age gradient is observed in half of the sample galaxies. The old component is slightly more metal poor than the young one. The metallicity gradient is negative and slightly positive in the old and young components, respectively. These results are in agreement with an inside-out scenario of disc formation and suggest a reduced impact of the radial migration on the stellar populations of the disc. The young component could be the result of a second burst of star formation in gas captured from the environment.