CHEMICAL EVOLUTION OF GALAXIES AND QUASAR METALLICITIES

As part of a continuing investigation of the connection between observational properties of active galactic nuclei and their host galaxies, we study the evolution of the chemical composition of the gas lost by stars in elliptical galaxies and bulges of spirals by means of a self-consistent model of galactic evolution. It is found that with Salpeter-like, or flatter, initial mass functions, solar abundances are reached in a few 10(8) yr, thereby explaining in a natural way the standard emission lines observed in high-redshift quasars. Furthermore, after the first Gyr, the same mass functions give rise to overabundances of N, Si, and Fe, and roughly solar abundances of C, Ne, O, and Mg, only weakly time-dependent. These results are consistent with the (few) available abundance estimates for the gas emitting the broad lines observed in quasar spectra and, in particular, they could solve the puzzle of the quasi-similarity of quasar spectra at different redshifts. Finally, the metal overabundances inferred in high-redshift quasars are inconsistent with the association of these objects with newly formed galaxies as prescribed, for example, by the starburst model of active galactic nuclei, since in this case quasars should have abundance ratios very different from the estimated ones as well as lower absolute abundances. A criterion for distinguishing between the early burst and other hypothesis is given on the basis of the predicted abundance ratios.