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

TL;DR

This study introduces accretion-modified stars (AMS) as a new stellar component in the chemical evolution of high-redshift galactic bulges hosting AGNs. It develops a closed-box chemical evolution framework that explicitly includes AMS growth and yields, and couples this with photoionization modeling (MAPPINGS V) to predict BPT diagram signatures. The results show that sustained AMS accretion can drive rapid, transient metal enrichment—up to several times solar—and produce distinctive abundance patterns and BPT trajectories, potentially observable in the early universe (e.g., z>15) with future spectroscopic surveys. The work highlights AMS as a plausible channel for early bulge enrichment and motivates further theoretical and observational efforts to detect non-canonical stellar populations in AGN host bulges, with nitrogen enrichment and very high-metallicity NLRs serving as potential indicators.

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.

Figures (11)

• Figure 1: The evolving AMS mass functions for a bulge with a total mass of $10^{10}\,M_\odot$, an SFE of 20 $\text{Gyr}^{-1}$, and a Salpeter ($\alpha$=2.35) IMF. The panels are arranged as follows: horizontal rows from left to right with increases of $\beta$ for a given $\dot{m}_\text{ac}^0$, and vertical with increases of $\dot{m}_\text{ac}^0$ from upper to below for a given $\beta$. The red and blue curves represent the initial and final (1 Gyr) stages of the evolution, respectively. The gray curves are uniformly distributed at a logarithmic interval of 0.3 dex in the range of $10^6-10^9\,$yr.
• Figure 2: Evolution of the rates of type II supernova for a bulge with a total mass of $10^{10}\,M_\odot$, an SFE of 20 $\text{Gyr}^{-1}$, and a Salpeter ($\alpha$=2.35) IMF. The panels are arranged as follows: horizontal rows from left to right with increases of $\beta$ for a given $\dot{m}_\text{ac}^0$, and vertical with increases of $\dot{m}_\text{ac}^0$ from upper to below for a given $\beta$. The solid and dashed curves represent the AMS and conventional GCE scenarios, respectively. The blue and red curves represent LIMs and CCSNe, respectively.
• Figure 3: Time-dependence of metallicity for a bulge with a total mass of $10^{10}\,M_\odot$, an SFE of 20 $\text{Gyr}^{-1}$, and a Salpeter ($\alpha$=2.35) IMF. Gray, blue, and red curves denote conventional GCE scenario, AMS with low-$\dot{m}_\text{ac}^0$ and AMS with high-$\dot{m}_\text{ac}^0$, respectively. Solid, dashed, and dotted lines represent models with $\beta = 1.0$, 1.5, and 2.0, respectively.
• Figure 4: Time-dependence of [X/H] abundance ratios for $\alpha$-elements (O, Ne, Mg, Si, S, and Ca), C, N, and Fe in a bulge with a total mass of $10^{10}\,M_\odot$, an SFE of $20\,\text{Gyr}^{-1}$, and a Salpeter IMF. The panel layout same as Figure \ref{['fig:MF_2.35']}. The different curves denote different elements as indicated in the upper left panel. The abundance of each element in the solar is as follows: $\rm [C/H]_{\odot}=-3.57$, $\rm [N/H]_{\odot}=-4.17$, $\rm [O/H]_{\odot}=-3.31$, $\rm [Ne/H]_{\odot}=-4.07$, $\rm [Mg/H]_{\odot}=4.4$, $\rm [Si/H]_{\odot}=-4.49$, $\rm [S/H]_{\odot}=-4.88$, $\rm [Ca/H]_{\odot}=-5.66$, and $\rm [Fe/H]_{\odot}=-4.5$.
• Figure 5: Time-dependence of [He/H] abundance ratios in a bulge with a total mass of $10^{10}\,M_\odot$, an SFE of $20\,\text{Gyr}^{-1}$, and a Salpeter IMF. The different curves denote different AMS parameters as indicated in the upper right panel. The He abundance in the solar is $\rm [He/H]_{\odot}=-1.07$.