Chemical evolution of bulges of active galactic nuclei in the early Universe: roles of accreting stars

TL;DR

This paper models how accreting stars in high-redshift galactic bulges influence chemical evolution, predicting rapid metal enrichment and distinctive emission-line signatures in AGN environments, with implications for future observations.

Contribution

Introduces the concept of accretion-modified stars (AMS) and models their impact on the chemical evolution of AGN host bulges at high redshift.

Findings

Rapid gas-phase metal enrichment up to five times solar within 0.1 Gyr.

High accretion rates cause shifts in emission-line diagnostics toward high-metallicity regimes.

Predicted abundance patterns and BPT diagram signatures can be tested with future spectroscopic data.

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 star (AMS) 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 AMS is modeled within 1 Gyr by combining population evolution and galactic chemical evolution formalisms, 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 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. Super-solar narrow-line regions observed in AGNs at z>15 may reflect such AMS-driven gas-phase enrichment of 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 non-canonical stellar populations in AGN host bulges.