Limits on Hierarchical Black Hole Mergers from the Most Negative χ _eff Systems

Abstract

It has been proposed that some black holes (BHs) in binary black hole (BBH) systems are born from “hierarchical mergers” (HMs), i.e., earlier mergers of smaller BHs. These HM products have spin magnitudes χ ∼ 0.7, and, if they are dynamically assembled into BBH systems, their spin orientations will sometimes be antialigned with the binary orbital angular momentum. In fact, as Baibhav et al. showed, ∼16% of BBH systems that include HM products will have an effective inspiral spin parameter, χ eff < −0.3. Nevertheless, the LIGO–Virgo–KAGRA (LVK) gravitational-wave (GW) detectors have yet to observe a BBH system with χ eff ≲ −0.2, leading to upper limits on the fraction of HM products in the population. We fit the astrophysical mass and spin distribution of BBH systems and measure the fraction of BBH systems with χ eff < −0.3, which implies an upper limit on the HM fraction. We find that fewer than 26% of systems in the underlying BBH population include HM products (90% credibility). Even among BBH systems with primary masses m 1 = 60 M ⊙, the HM fraction is less than 69%, which may constrain the location of the pair-instability mass gap. With 300 GW events (to be expected in the LVK’s next observing run), if we fail to observe a BBH with χ eff < −0.3, we can conclude that the HM fraction is smaller than 2.5 − 2.2 + 9.1 % .

Globular cluster formation histories, masses and radii inferred from gravitational waves

“…However, we may still be overestimating the contribution from GCs, because multiple other formation channels and environments can produce BBH mergers with 𝜒 eff < 0. In the future, we can use more specific criteria to isolate the GC contribution to the merger rate, including, for example, the BBH population distribution of orbital eccentricity (Samsing 2018;Rodriguez et al 2018a;Zevin et al 2019;Arca Sedda et al 2021), which can be used to measure the fraction of BBHs assembled in GCs (Zevin et al 2021b;Romero-Shaw et al 2022), and the rate of hierarchical mergers (Rodriguez et al 2018b;Kimball et al 2021;Gerosa & Fishbach 2021) as inferred from the distribution of spin magnitudes (Fishbach et al 2017;Baibhav et al 2020;Fishbach et al 2022). Given robust predictions for the BBH population distribution of eccentricity, spin magnitudes, and/or masses as a function of GC properties, we may incorporate these additional BBH observables into our inference, and improve our constraints on GC properties.…”

“…Nevertheless, GCs are unlikely to account for the total measured BBH merger rate (Zevin et al 2021a;Wong et al 2021;Mapelli et al 2022). The clearest indication for this is that BBH mergers that are dynamically assembled in GCs are expected to have isotropically-oriented spin directions (Rodriguez et al 2016b), while only a fraction of BBH systems are consistent with isotropic spins (Abbott et al 2021c;The LIGO Scientific Collaboration et al 2021b;Callister et al 2022;Tong et al 2022;Fishbach et al 2022). Therefore, we first use GWTC-3 to measure the rate of BBH mergers that are consistent with a GC origin in §3.…”

Quiescent and Active Galactic Nuclei as Factories of Merging Compact Objects in the Era of Gravitational Wave Astronomy

“…As suggested by the BBH mergers detected during the three LVC observation runs [141], this parameter seems to cluster around low value [e.g. [472][473][474][475][476].…”

Potential Subpopulations and Assembling Tendency of the Merging Black Holes

“…For a coalescing system with nonspinning equal-mass components, the remnant will have a spin amplitude ∼0.7; on the one hand, nonzero and isotropically distributed spins of 1G BHs (due to, e.g., natal kicks) will increase the dispersion of the spin amplitude distribution of 2G mergers; on the other hand, if the component spins and orbital angular momentum were aligned by the accretion torque and corotation torques in the AGN disk, the remnants could have larger spins. Recently, Fishbach et al (2022) addressed the limits on hierarchical black hole mergers from the most negative χ eff systems; their work mainly focuses on classical star clusters, and further studies can be carried out to quantitatively constrain the mergers in an AGN disk.…”