Outliers in the LIGO black hole mass function from coagulation in dense clusters

Abstract

The advanced LIGO O3a run catalog has been recently published, and it includes several events with unexpected mass properties, including mergers with individual masses in the lower and upper mass gaps, as well as mergers with unusually small mass ratios between the binary components. Here we entertain the possibility that these outliers are the outcome of hierarchical mergers of black holes or neutron stars in the dense environments of globular clusters. We use the coagulation equation to study the evolution of the black hole mass function within a typical cluster. Our prescription allows us to monitor how various global quantities change with time, such as the total mass and number of compact objects in the cluster, its overall merger rate, and the probability to form intermediate-mass black holes via a runaway process. By accounting for the LIGO observational bias, we predict the merger event distributions with respect to various variables such as the individual masses M1 and M2, their ratio q, and redshift z, and we compare our predictions with the published O3a data. We study how these distributions depend on the merger-rate and ejections parameters and produce forecasts for the (tight) constraints that can be placed on our model parameters using the future dataset of the O5 run. Finally, we also consider the presence of a static channel with no coagulation producing merger events alongside the dynamic channel, finding that the two can be distinguished based solely on the merger mass distribution with future O5 data.

The Earliest Stage of Galactic Star Formation

“…Nevertheless, studies focusing on nearly every individual region of the stellar mass function appear to report an excess when compared with expectations taken from a Galactic IMF. Black hole merger rates from LIGO find an excess compared to predictions using a Galactic IMF, consistent with a top-heavier IMF that produced more black holes (Kovetz et al 2018;Flitter et al 2021) Each of these results, in addition to probing a different stellar mass range, also probes a different time period. Thus, an evolutionary track that moves from an initial, bottom-light IMF to a progressively bottom-heavier IMF over time is potentially consistent with all of these results.…”

The Astro-Primordial Black Hole Merger Rates: a Reappraisal

“…We also write M cl = 2M h for the cluster's total mass. For simplicity, we neglect the evolution of clusters through time assuming all GCs to have a lifetime of T cl 10 Gyr [62,65,66], and the spin of the BHs in the analysis. The number density of globular clusters (GCs) in the local universe is estimated using the conventional value of n GC = 2.4 +0.9 −0.9 Mpc −3 [38,54,[66][67][68].…”

“…For simplicity, we neglect the evolution of clusters through time assuming all GCs to have a lifetime of T cl 10 Gyr [62,65,66], and the spin of the BHs in the analysis. The number density of globular clusters (GCs) in the local universe is estimated using the conventional value of n GC = 2.4 +0.9 −0.9 Mpc −3 [38,54,[66][67][68]. In any case, our final results for the merger rate density will scale proportional to this number density.…”

Prospects of Identifying Hierarchical Triple Mergers for the Third-generation Ground-based Detectors

“…This process may constitute a significant mechanism for the formation of intermediate-mass black holes (BHs) in globular clusters (Miller & Hamilton 2002). Extensive research has been conducted on the hierarchical merger scenario, including studies on the modeling of BBH populations with hierarchical mergers (Doctor et al 2020), the distribution of spin magnitudes resulting from hierarchical mergers (Fishbach et al 2017), the coagulation processes within the dense environments of globular clusters that may explain mergers spanning the lower and upper mass gaps (Flitter et al 2021), the differentiation between field and dynamical capture formation scenarios (Gerosa & Berti 2017), and the simulation of hierarchical mergers across various environments (Mapelli et al 2021). The fourth and fifth observing runs of aLIGO/ AdV are expected to increase the inventory of compact binary coalescences, enhancing the detections of BHs that have formed through hierarchical mergers.…”