Abstract
Abstract The astrophysical origin of gravitational-wave (GW) events is one of the most timely problems in the wake of the LIGO/Virgo discoveries. In active galactic nuclei (AGNs), binaries form and evolve efficiently by dynamical interactions and gaseous dissipation. Previous studies have suggested that binary black hole (BBH) mergers in AGN disks can contribute significantly to BBH mergers observed by GW interferometers. Here we examine the distribution of the effective spin parameter χ eff of this GW source population. We extend our semi-analytical model of binary formation and evolution in AGN disks by following the evolution of the binary orbital angular momenta and black hole (BH) spins. BH spins change due to gas accretion and BH mergers, while the binary orbital angular momenta evolve due to gas accretion and binary–single interactions. We find that the distribution of χ eff predicted by our AGN model is similar to the distribution observed during LIGO/Virgo O1 and O2. On the other hand, if radial migration of BHs is inefficient, χ eff is skewed toward higher values than the observed distribution, because of the paucity of scattering events that would randomize spin directions relative to the orbital plane. We suggest that high binary masses and the positive correlation between binary mass and the standard deviation of χ eff for chirp masses up to can be possible signatures for mergers originating in AGN disks. Finally, hierarchical mergers in AGN disks naturally produce properties of the recent GW event GW190412, including a low mass ratio, a high primary BH spin, and a significant spin component in the orbital plane.
Highlights
Recent detections of gravitational waves (GWs) have shown evidence for a high rate of black hole (BH)–BH and neutron star (NS)–NS mergers in the universe (The LIGO Scientific Collaboration et al 2019; Venumadhav et al 2019)
Focusing on mergers occurring outside migration traps, McKernan et al (2019) estimated the mass and spin distribution for mergers among binaries formed during close encounters in active galactic nucleus (AGN) disks, by assuming that binaries are always aligned or anti-aligned with the AGN disk, and BH mergers are much faster than the growth of BH spins by gas accretion
There are two types of BHs differentiated by their origin: BHs formed before the beginning of the current AGN phase and BHs formed during the current AGN phase (BHs formed in situ)
Summary
Recent detections of gravitational waves (GWs) have shown evidence for a high rate of black hole (BH)–BH and neutron star (NS)–NS mergers in the universe (The LIGO Scientific Collaboration et al 2019; Venumadhav et al 2019). Focusing on mergers occurring outside migration traps, McKernan et al (2019) estimated the mass and spin distribution for mergers among binaries formed during close encounters in AGN disks, by assuming that binaries are always aligned or anti-aligned with the AGN disk, and BH mergers are much faster than the growth of BH spins by gas accretion They found that the distribution of χeff is symmetric around zero, and the dispersion of χeff is determined by the magnitude of initial BH spins. To determine the χeff distribution for mergers in AGN disks, it is necessary to follow the orbital angular momenta of the binaries, again taking into account both binary–single interactions and gas accretion.
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