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

TL;DR

This paper suggests that a top-heavy initial mass function in galactic nuclei, driven by dense AGN disk conditions, explains the rapid iron enrichment observed in high-redshift AGNs, with implications for stellar populations and gravitational wave sources.

Contribution

It introduces a model where top-heavy stellar IMFs in AGN disks account for high [Fe/Mg] ratios and predicts specific IMF parameters and stellar mass cutoffs consistent with observations.

Findings

High [Fe/Mg] ratios imply a top-heavy IMF with Gamma    -1 and M_max e 150 M_\u00b0.

Core-collapse and pair-instability supernovae significantly contribute to iron enrichment.

Massive stellar populations in AGN disks produce black hole remnants, explaining a fraction of LIGO/Virgo gravitational-wave events.

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 ${\rm [Fe/Mg]} \gtrsim 0.2$ 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.