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Recent observations of active galactic nuclei (AGNs) have shown a highFe~II/Mg~II line-flux ratio in their broad-line regions, nearly independent ofredshift up to $z \gtrsim 6$. The high flux ratio requires rapid production ofiron in galactic nuclei to reach an abundance ratio of ${\rm [Fe/Mg]} \gtrsim0.2$ as high as those observed in matured galaxies in the local universe. Wepropose a possible explanation of rapid iron enrichment in AGNs by massive starformation that follows a top-heavy initial mass function (IMF) with a power-lawindex of $\Gamma$ larger than the canonical value of $\Gamma=-2.35$ for aSalpeter IMF. Taking into account metal production channels from differenttypes of SNe, we find that the high value of ${\rm [Fe/Mg]} \gtrsim 0.2$requires the IMF to be characterized with $\Gamma \gtrsim -1$ ($\Gamma \gtrsim0$) 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-collapseSNe with $M_\ast \gtrsim 70~{\rm M_\odot}$ and pair-instability SNe give amajor contribution for iron enrichment. Such top-heavy stellar IMFs would be anatural consequence from mass growth of stars formed in dense AGN disks underBondi-like gas accretion that is regulated by feedback at $M_\ast \gtrsim10~{\rm M_\odot}$. The massive stellar population formed in AGN disks alsoleave stellar-mass black hole remnants, whose mergers associated withgravitational-wave emission account for at most 10 \% of the merger rateinferred from LIGO/Virgo observations to simultaneously explain the high ${\rm[Fe/Mg]}$ ratio with metal ejection.

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