Chemical Evolution of Bulges of Active Galactic Nuclei in the Early Universe: Roles of Accreting Stars

Abstract JWST/NIRCam observations reveal dense stellar cores in high-redshift galactic bulges, indicative of sustained star formation and potential stellar accretion. We introduce accretion-modified stars (AMSs) as a new component in the chemical evolution of high-redshift bulges hosting active galactic nuclei (AGNs). The gas-phase chemical evolution of bulge environments containing AMSs is modeled within 1 Gyr by combining formalisms for population evolution and galactic chemical evolution, and observational signatures are tracked via photoionization modeling on Baldwin–Phillips–Terlevich (BPT) diagrams. Sustained high accretion onto AMSs leads to rapid gas-phase metal enrichment of the bulge, producing abundance peaks up to five times solar metallicity within 0.1 Gyr and significantly modifying elemental ratios in the gas phase. Atypical gas-phase abundance patterns occur during early high-accretion phases and gradually diminish as the accretion rate declines. In BPT diagrams, high-AMS-accretion scenarios shift the modeled emission-line sequence toward the local AGN branch and extend into the high-metallicity regime. Supersolar narrow-line regions observed in AGNs at z ≳ 15 may reflect such AMS-driven gas-phase enrichment of the host bulge under extreme gas densities. While direct detection of AMSs within AGN bulges remains challenging, the model provides testable predictions for future spectroscopic surveys and motivates further exploration of noncanonical stellar populations in AGN host bulges.