Mass-gap Mergers in Active Galactic Nuclei

Hiromichi Tagawa,Bence Kocsis,Imre Bartos,Kazuyuki Omukai,Zoltán Haiman,Johan Samsing

doi:10.3847/1538-4357/abd555

Hiromichi Tagawa, Bence Kocsis + Show 4 more

Open Access

https://doi.org/10.3847/1538-4357/abd555

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Abstract

Abstract The recently discovered gravitational wave sources GW190521 and GW190814 have shown evidence of BH mergers with masses and spins outside of the range expected from isolated stellar evolution. These merging objects could have undergone previous mergers. Such hierarchical mergers are predicted to be frequent in active galactic nuclei (AGNs) disks, where binaries form and evolve efficiently by dynamical interactions and gaseous dissipation. Here we compare the properties of these observed events to the theoretical models of mergers in AGN disks, which are obtained by performing one-dimensional N-body simulations combined with semi-analytical prescriptions. The high BH masses in GW190521 are consistent with mergers of high-generation (high-g) BHs where the initial progenitor stars had high metallicity, 2g BHs if the original progenitors were metal-poor, or 1g BHs that had gained mass via super-Eddington accretion. Other measured properties related to spin parameters in GW190521 are also consistent with mergers in AGN disks. Furthermore, mergers in the lower mass gap or those with low mass ratio as found in GW190814 and GW190412 are also reproduced by mergers of 2g–1g or 1g–1g objects with significant accretion in AGN disks. Finally, due to gas accretion, the massive neutron star merger reported in GW190425 can be produced in an AGN disk.

Highlights

Several gravitational wave (GW) events were reported by LIGO and Virgo whose measured physical properties pose interesting constraints on their astrophysical origin
The distributions are weighted by the observable volume depending on the masses of merging binaries calibrated for a LIGO Handford prior to O3 as in Paper I, while the dependence on either χeff or eccentricity is ignored in terms of detection volume for simplicity
Since there are uncertainties in the properties of the AGN disks and the compact objects (COs) distributions, and they are expected to vary from one galaxy to another, we investigated the dependence of the results on the gas accretion rate from outer radius of the simulation, the size of the AGN disks, the mass of the central supermassive BH (SMBH), the initial mass function of COs, the stellar mass enclosed within 3 pc, the initial density profile of COs, and the initial velocity dispersion of COs in the Appendix

Summary

Introduction

Several gravitational wave (GW) events were reported by LIGO and Virgo whose measured physical properties pose interesting constraints on their astrophysical origin. One of these is GW190521 (Abbott et al 2020d; The. LIGO Scientific Collaboration et al 2020b), in which the masses of the merging BHs (85-+1241 M and 66-+1187 M , but see Nitz & Capano 2021) indicate that the merging BHs fall into the so-called “upper-mass gap,” as they are heavier than the maximum BH mass imposed by (pulsation) pair-instability supernovae (Chatzopoulos & Wheeler 2012; Farmer et al.2019). A possible explanation proposed for the large masses in these events is isolated binary evolution, presuming that massive BHs can form from

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