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 indicate 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 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; and 20\% of which is produced by NDAF outflows) at z 8 for the ``top-heavy'' IMF model in Toyouchi2022, which readily explains the quasar observations on the super-solar metal abundance and redshift-independent evolution.