Recent observations of active galactic nuclei (AGNs) have shown a high Fe~II/Mg~II line-flux ratio in their broad-line regions, nearly independent of redshift up to $z \gtrsim 6$. The high flux ratio requires rapid production of iron in galactic nuclei to reach an abundance ratio of ${\rm [Fe/Mg]} \gtrsim 0.2$ as high as those observed in matured galaxies in the local universe. We propose a possible explanation of rapid iron enrichment in AGNs by massive star formation that follows a top-heavy initial mass function (IMF) with a power-law index of $Γ$ larger than the canonical value of $Γ=-2.35$ for a Salpeter IMF. Taking into account metal production channels from different types of SNe, we find that the high value of ${\rm [Fe/Mg]} \gtrsim 0.2$ requires the IMF to be characterized with $Γ\gtrsim -1$ ($Γ\gtrsim 0$) and a high-mass cutoff at $M_{\rm max} \simeq 100$--$150~{\rm M_\odot}$ $(M_{\rm max} \gtrsim 250~{\rm M_\odot})$. Given the conditions, core-collapse SNe with $M_\ast \gtrsim 70~{\rm M_\odot}$ and pair-instability SNe give a major contribution for iron enrichment. Such top-heavy stellar IMFs would be a natural consequence from mass growth of stars formed in dense AGN disks under Bondi-like gas accretion that is regulated by feedback at $M_\ast \gtrsim 10~{\rm M_\odot}$. The massive stellar population formed in AGN disks also leave stellar-mass black hole remnants, whose mergers associated with gravitational-wave emission account for at most 10 \% of the merger rate inferred from LIGO/Virgo observations to simultaneously explain the high ${\rm [Fe/Mg]}$ ratio with metal ejection.
Mid-Infrared Outbursts in Nearby Galaxies: Nuclear Obscuration and Connections to Hidden Tidal Disruption Events and Changing-Look Active Galactic Nuclei
The Effect of Star--Disk Interactions on Highly Eccentric Stellar Orbits in Active Galactic Nuclei: A Disk Loss Cone and Implications for Stellar Tidal Disruption Events
Anisotropic Radiation in Super-Eddington Active Galactic Nuclei: The Responsivity of Broad-Line Regions to Continuum Variability
State Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences（高能物理研究所粒子天体物理国家重点实验室）；National Astronomical Observatories, Chinese Academy of Sciences（中国科学院国家天文台）；School of Astronomy and Space Science, University of Chinese Academy of Sciences（中国科学院大学天文与空间科学学院）
Active galactic nuclei do not exhibit strictly sinusoidal brightness variations
California Institute of Technology（加州理工学院）；Max Planck Institute for Astronomy（马克斯·普朗克天文学研究所）；Center for Computational Astrophysics, Flatiron Institute（平顿研究所计算天体物理中心）；New York University（纽约大学）
Unification Model of Active Galactic Nuclei by Photoionization Equilibrium Calculation Based on Radiative Hydrodynamic Simulations
Graduate School of Science and Engineering, Kagoshima University（鹿儿岛大学理工学研究科）；Research Center for Space and Cosmic Evolution, Ehime University（爱媛大学宇宙 evolution 研究中心）；Faculty of Science, Hokkaido University（北海道大学理学部）；Astronomical Institute, Tohoku University（东北大学天文学研究所）；Faculty of Natural Sciences, National Institute of Technology, Kure College（国立神户高等专门学校自然科学学部）；Graduate School of Science and Technology, Hirosaki University（弘前大学理工学研究科）；Department of Earth and Space Science, The University of Osaka（大阪大学地球空间科学系）
Large-scale environments of star-forming active galactic nuclei: How black hole mass, accretion rate, and luminosity connect to dark matter halos
Instituto de Fisica de Cantabria (CSIC-Universidad de Cantabria)（坎塔布里亚物理研究所（西班牙科学研究中心-坎塔布里亚大学））；Department of Physics and Astronomy, University of Southampton（南安普顿大学物理与天文系）；Institute for Astronomy & Astrophysics, National Observatory of Athens（雅典国家天文台天体物理学与天文学研究所）
Standardizing reverberation-mapped H$α$ and H$β$ active galactic nuclei using radius--luminosity relations involving monochromatic and broad H$α$ luminosities
Standardizing a larger, higher-quality, homogeneous sample of reverberation-mapped H$β$ active galactic nuclei using the broad-line region radius-luminosity relation
