Oral Presentation

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

Author(s): Daisuke Toyouchi (Tokyo Univ.)

Presenter: Daisuke Toyouchi (The University of Tokyo)

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 ~ 7. The high flux ratio requires rapid production of iron in galactic nuclei to reach an abundance ratio of [Fe/Mg] ~ 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). Taking into account metal production channels from different types of SNe, we find that the high value of [Fe/Mg] > 0.2 requires the top-heavy IMF with a power-law index Gamma > -1 and a high-mass cutoff at Mmax = 100-150 Msun (Gamma = -2.35 and Mmax < 100 Msun for a canonical Salpeter IMF). Given the conditions, core-collapse SNe with Ms > 70 Msun give a dominant contribution to iron enrichment. Such top-heavy stellar IMFs would naturally result from rapid gas accretion onto stars formed in dense AGN disks. In this talk, we also argue the cosmic rate of binary BH mergers based on the top-heavy IMF hypothesis.