Signatures of hierarchical mergers in black hole spin and mass distribution

Abstract

Recent gravitational wave (GW) observations by LIGO/Virgo show evidence for hierarchical mergers, where the merging BHs are the remnants of previous BH merger events. These events may carry important clues about the astrophysical host environments of the GW sources. In this paper, we present the distributions of the effective spin parameter (χeff), the precession spin parameter (χp), and the chirp mass (mchirp) expected in hierarchical mergers. Under a wide range of assumptions, hierarchical mergers produce (i) a monotonic increase of the average of the typical total spin for merging binaries, which we characterize with ${\bar{\chi }}_\mathrm{typ}\equiv \overline{(\chi _\mathrm{eff}^2+\chi _\mathrm{p}^2)^{1/2}}$, up to roughly the maximum mchirp among first-generation (1g) BHs, and (ii) a plateau at ${\bar{\chi }}_\mathrm{typ}\sim 0.6$ at higher mchirp. We suggest that the maximum mass and typical spin magnitudes for 1g BHs can be estimated from ${\bar{\chi }}_\mathrm{typ}$ as a function of mchirp. The GW data observed in LIGO/Virgo O1–O3a prefers an increase in ${\bar{\chi }}_\mathrm{typ}$ at low mchirp, which is consistent with the growth of the BH spin magnitude by hierarchical mergers, at ∼2σ confidence. A Bayesian analysis using the χeff, χp, and mchirp distributions suggests that 1g BHs have the maximum mass of ∼15–30 M⊙ if the majority of mergers are of high-generation BHs (not among 1g-1g BHs), which is consistent with mergers in active galactic nucleus disks and/or nuclear star clusters, while if mergers mainly originate from globular clusters, 1g BHs are favored to have non-zero spin magnitudes of ∼0.3. We also forecast that signatures for hierarchical mergers in the ${\bar{\chi }}_\mathrm{typ}$ distribution can be confidently recovered once the number of GW events increases to ≳ O(100).

The population of merging compact binaries inferred using gravitational waves through GWTC-3

“…Moreover, BHs formed from progenitor stars at low metallicities (Z/Z 0.1) might avoid all together the mass limit imposed by pair-instability [175,244,245]. The lack of a sharp truncation at high masses might indicate a dynamical process, such as the hierarchical merger of BHs [109,152,196,197,199,222,227,231,[247][248][249][250][251] or stars [252][253][254][255] in dense clusters or in the gaseous disk surrounding a massive BH [206]. In a hierarchical scenario we would expect the more massive BHs to also have the larger spins [196,197].…”

Spin Evolution of Stellar-mass Black Holes Embedded in AGN disks: Orbital Eccentricity Produces Retrograde Circumstellar Flows

“…The effective spin parameter χ eff measured from a merger event, which is the mass-weighted-average of the sBH spins projected along the BBH orbital angular momentum axis, can help constrain compact-object merger pathways (e.g., Gerosa et al 2018;Bavera et al 2020;Wang et al 2021b;Tagawa et al 2020aTagawa et al , 2021a. The χ eff distribution inferred from the observed GW events prefers low values on average (The LIGO Scientific Collaboration et al 2021), which suggests low natal sBH spins or misalignment between the binary orbital angular momentum and sBH spins (Farr et al 2017).…”

"Super-Kilonovae" from Massive Collapsars as Signatures of Black-Hole Birth in the Pair-instability Mass Gap

“…These unusual properties have motivated a number of theoretical studies proposing new ways to populate the PI mass gap, such as through dynamical stellar mergers (e.g. Di Carlo et al 2019Carlo et al , 2020Renzo et al 2020a), hierarchical black hole mergers in dense environments (e.g., Antonini & Rasio 2016;Yang et al 2019;Tagawa et al 2021;Gerosa & Fishbach 2021), modifying stellar physics at low metallicity (e.g. Farrell et al 2021;Vink et al 2021), or through external gas accre-tion (e.g., Safarzadeh & Haiman 2020).…”