Kavli Affiliate: Kohei Inayoshi
| First 5 Authors: Hanpu Liu, Kohei Inayoshi, , ,
| Summary:
Massive black holes (BHs) grow by gas accretion and mergers, observable
through electromagnetic (EM) and gravitational wave (GW) emission. The James
Webb Space Telescope (JWST) has detected faint active galactic nuclei (AGNs),
revealing an abundant population of accreting BHs with masses of $M_bulletsim
10^{6-8}~M_odot$. This mass range overlaps with the detection scopes of
space-based GW interferometers and approaches the upper bounds of the predicted
mass of seed BHs. We model BH mass assembly in light of the new JWST findings
to investigate their formation channels and predict merger events. Two types of
seed BHs are considered: heavy seeds ($M_bulletsim 10^{2-5}~M_odot$) formed
in rare and overdense cosmic regions, and light seeds ($M_bulletsim
10^{1-3}~M_odot$) formed as stellar remnants in less massive dark-matter
halos. The BHs grow through episodic accretion and merger events, which we
model by fitting the AGN luminosity function to observational data including
JWST-identified AGNs at $zsim 5$. We find that heavy seeds alone struggle to
explain quasars and faint JWST-selected AGNs simultaneously, requiring the more
abundant light seeds. The observed merger rate of BHs from heavy seeds alone is
limited to $lesssim 10^{-1}~{rm yr}^{-1}$ for major mergers at $zgeq5$.
However, the presence of light seeds increases the major merger rate by several
orders of magnitude, which peaks at a total BH mass of $M_bulletsimeq 2times
10^3~M_odot$ over $5<z<10$ at a rate of $sim 30~{rm yr}^{-1}$. These events
are detectable by future GW observatories such as the Laser Interferometer
Space Antenna (LISA). Precise sky localization and distance measurement of
those GW events, with solid angle and luminosity distance uncertainties
$DeltaOmegaDeltalog D_Llesssim 10^{-4}~rm deg^2$, will enable EM
identification of mergers at $zgeq5$ and multi-messenger follow-up
observations.
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