AGN Disks Harden the Mass Distribution of Stellar-mass Binary Black Hole Mergers

Abstract

The growing number of stellar-mass binary black hole mergers discovered by Advanced LIGO and Advanced Virgo are starting to constrain the binaries' origin and environment. However, we still lack sufficiently accurate modeling of binary formation channels to obtain strong constraints, or to identify sub-populations. One promising formation mechanism that could result in different black hole properties is binaries merging within the accretion disks of Active Galactic Nuclei (AGN). Here we show that the black holes' orbital alignment with the AGN disks preferentially selects heavier black holes. We carry out Monte Carlo simulations of orbital alignment with AGN disks, and find that AGNs harden the initial black hole mass function. Assuming an initial power law mass distribution $M_{\mathrm{bh}}^{-\beta }$, we find that the power law index changes by $\Delta \beta \sim 1.3$, resulting in a more top-heavy population of merging black holes. This change is independent of the mass of, and accretion rate onto, the supermassive black hole in the center of the AGN. Our simulations predict an AGN-assisted merger rate of $\sim 4$Gpc${}^{-3}$yr${}^{-1}$. With its hardened mass spectra, the AGN channel could be responsible for $10-50$% of gravitational-wave detections.

accepted to ApJ; 10 pages, 6 figures

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• Polluting the pair-instability mass gap for binary black holes through super-Eddington accretion in isolated binaries
• Fingerprints of binary black hole formation channels encoded in the mass and spin of merger remnants
• Black Hole Formation in the Lower Mass Gap through Mergers and Accretion in AGN Disks
• GW190412 as a Third-Generation Black Hole Merger from a Super Star Cluster
• Effective spin distribution of black hole mergers in triples
• GW170817A as a Hierarchical Black Hole Merger