Top-heavy stellar mass distribution in galactic nuclei inferred from the universally high abundance ratio of [Fe/Mg]

Kavli Affiliate: Kohei Inayoshi

| First 5 Authors: Daisuke Toyouchi, Kohei Inayoshi, Miho N. Ishigaki, Nozomu Tominaga,

| Summary:

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 $Gamma$ larger than the canonical value of $Gamma=-2.35$ 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 $Gamma gtrsim -1$ ($Gamma gtrsim
0$) 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.

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