Evolution of chemical abundances in Seyfert galaxies

Abstract

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 × 10 9 −10 11 M 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 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 ∼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.

Chemical evolution of local galaxies in a hierarchical model

“…We use this value to estimate the stellar mass in the disc of our selected MW‐like galaxies, which is, since all the cold gas is in the disc, M *,d = M g /0.2. By assuming that the stellar mass of the MW bulge is 1–2 × 10 10 M ⊙ (Reshetnikov 2000; Ballero et al 2008), the observed disc‐to‐total stellar mass ratio is M *,d / M *,tot = 0.7–0.8. Of the MW‐like galaxies selected on the basis of their gas accretion history, we now select those with values ∼0.7–0.8.…”

Anatomy of the AGN in NGC 5548

“…Furthermore, the theoretical expectations for the value of chemical abundances in an AGN environment as a function of metalicity are highly varied. The leading models can differ about relative abundances values by factors of three or more (e.g., comparing the values of Hamann & Ferland 1993;Ballero et al 2008). …”

Near-infrared spectroscopy of a nitrogen-loud quasar SDSS J1707+6443

“…Note that the measurement of the N vλ1240 flux is sometimes uncertain because of the heavy blending with the Lyα emission (see, e.g., Nagao et al 2006b); however, the N-loudness of these quasars is convincing because the wellisolated N iv]λ1486 and N iii]λ1750 are used for defining the N-loud quasar population. Standard chemical evolution models cannot predict these extremely high metallicities at any epoch (e.g., Ballero et al 2008), therefore these N-loud quasars is a great challenge for galaxy evolutionary models. Jiang et al (2008) pointed out that N-loud quasars may not have such high Z BLR , but simply have an unusually high relative abundance of nitrogen in the BLR, mainly because the emission-line spectrum of N-loud quasars is not significantly different from that of typical quasars except for the nitrogen lines.…”