2021/12/12 by Daisuke Toyouchi, Kohei Inayoshi, Miho N. Ishigaki +1
Physics and Astronomy · #Accretion (finance) #Active galactic nucleus #Astronomy #Astronomy and Astrophysical Research #Astrophysical Phenomena and Observations #Astrophysics #Flux (metallurgy) #Galaxies: Formation, Evolution, Phenomena #Galaxy #Gamma-ray bursts and supernovae #Initial mass function #Physics #Population #Redshift #Star formation #Stars #Stellar mass #astro-ph.GA
paper · pdf · open access · doi:10.1093/mnras/stac640
published in Monthly Notices of the Royal Astronomical Society 512(2), 2573-2583 (Oxford University Press) · 9 pages, 5 figures, submitted to MNRAS
arxiv created 2021/12/12 · openalex created_date 2021/12/31 · openalex publication_date 2022/03/08 · arxiv updated 2022/03/14 · openalex updated_date 2026/08/01
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 ≃ 100--150~\rm M_\odot (M\rm max \gtrsim 250~\rm M_\odot). Given the conditions, core-collapse SNe with M_∗ \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_∗ \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.