Nucleosynthesis Contribution of Neutrino-dominated Accretion Flows to the Chemical Evolution of Active Galactic Nuclei

Abstract

Recent observations of quasars show high line-flux ratios in their broad emission lines and the ratios appear to be independent of redshift up to z ≳ 6, which indicates that the broad-line regions of these early quasars are surprisingly metal-rich. Here, we revisit the chemical evolution of high-redshift quasars by adding a new ingredient, i.e., the neutrino-dominated accretion flows (NDAFs) with outflows, on top of the conventional core-collapse supernovae (CCSNe). In the presence of the chemical contribution from NDAFs with outflows, the total metal mass (i.e., the summation of the conventional CCSN and NDAFs with outflows) per CCSN depends weakly upon the mass of the progenitor star if the mass is in the range of ∼25–55 M ⊙. We model the chemical evolution by adopting a improved open-box model with three typical initial mass functions (IMFs). We find that, with the additional chemical contribution from NDAFs with outflows, the quasar metallicity can be enriched more rapidly in the very early universe (z ∼ 10) and reaches a higher saturation than the no-NDAF case at z ∼ 8, after which they evolve slowly with redshift. The quasar metallicity can reach ∼20 Z ⊙ (Z ⊙ denotes the metallicity of the Sun, ∼20% of which is produced by NDAF outflows) at z ∼ 8 for the “top-heavy” IMF model in Toyouchi et al., which readily explains the quasar observations on the supersolar metal abundance and redshift-independent evolution.