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

Abstract

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 $[Fe/Mg]\gtrsim 0.2$ 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 $[Fe/Mg]\gtrsim 0.2$ requires the IMF to be characterized with $\Gamma \gtrsim -1$ ($\Gamma \gtrsim 0$) and a high-mass cutoff at $M_{\max }\simeq 100$--$150{\mathrm{M}}_{\odot }$ $(M_{\max }\gtrsim 250{\mathrm{M}}_{\odot })$. Given the conditions, core-collapse SNe with $M_{\ast }\gtrsim 70{\mathrm{M}}_{\odot }$ 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 $M_{\ast }\gtrsim 10{\mathrm{M}}_{\odot }$. 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 $[Fe/Mg]$ ratio with metal ejection.

9 pages, 5 figures, submitted to MNRAS

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