Evolution of chemical abundances in Seyfert galaxies

Aims. We study the chemical evolution of spiral bulges hosting Seyfert nuclei, based on updated chemical and spectro-photometrical evolution models for the bulge of our Galaxy, to make predictions about other quantities measured in Seyferts and to model the photometric features of local bulges. The chemical evolution model contains updated and detailed calculations of the Galactic potential and of the feedback from the central supermassive black hole, and the spectro-photometric model covers a wide range of stellar ages and metallicities. Methods. We computed the evolution of bulges in the mass range 2 x 10(9)-10(11) M-circle dot by scaling the efficiency of star formation and the bulge scalelength, as in the inverse-wind scenario for elliptical galaxies, and by considering an Eddington limited accretion onto the central supermassive black hole. Results. We successfully reproduced the observed relation between the masses of the black hole and of the host bulge. The observed nuclear bolometric luminosity emitted by the supermassive black hole is reproduced only at high redshift or for the most massive bulges; in the other cases, a rejuvenation mechanism is necessary at z similar or equal to 0. The energy provided by the black hole is in most cases not significant for triggering the galactic wind. The observed high star-formation rates and metal overabundances are easily achieved, as are the constancy of chemical abundances with the redshift and present-day colours of bulges. Those results are not affected if we vary the index of the stellar IMF from x = 0.95 to x = 1.35. A steeper IMF is instead required in order to reproduce the colour-magnitude relation and the present K-band luminosity of the bulge. Conclusions. We show that the chemical evolution of the host bulge, with a short formation timescale of similar to 0.1 Gyr, a rather high efficiency of star formation ranging from 11 to 50 Gyr(-1) according to the bulge mass, and an IMF flatter than the solar neighbourhood, combined with the accretion onto the black hole, is sufficient to explain the main observed features of Seyfert galaxies.