Mass and Rate of Hierarchical Black Hole Mergers in Young, Globular and Nuclear Star Clusters

Abstract

Hierarchical mergers are one of the distinctive signatures of binary black hole (BBH) formation through dynamical evolution. Here, we present a fast semi-analytic approach to simulate hierarchical mergers in nuclear star clusters (NSCs), globular clusters (GCs) and young star clusters (YSCs). Hierarchical mergers are more common in NSCs than they are in both GCs and YSCs because of the different escape velocity. The mass distribution of hierarchical BBHs strongly depends on the properties of first-generation BBHs, such as their progenitor’s metallicity. In our fiducial model, we form black holes (BHs) with masses up to ∼103 M⊙ in NSCs and up to ∼102 M⊙ in both GCs and YSCs. When escape velocities in excess of 100 km s−1 are considered, BHs with mass >103 M⊙ are allowed to form in NSCs. Hierarchical mergers lead to the formation of BHs in the pair instability mass gap and intermediate-mass BHs, but only in metal-poor environments. The local BBH merger rate in our models ranges from ∼10 to ∼60 Gpc−3 yr−1; hierarchical BBHs in NSCs account for ∼10−2–0.2 Gpc−3 yr−1, with a strong upper limit of ∼10 Gpc−3 yr−1. When comparing our models with the second gravitational-wave transient catalog, we find that multiple formation channels are favored to reproduce the observed BBH population.

The GW190521 Mass Gap Event and the Primordial Black Hole Scenario

“…Even though no firm conclusion can be drawn due to the small SNR and the large uncertainties coming from different waveform models [96], this pattern seems to be in agreement with the parameters measured for GW190521. This gives strong motivation to reconsider the parameter estimation of this event incorporating the correct PBH-motivated prior distributions to infer the binary parameters in the PBH scenario [97], and to perform a Bayesian comparison between the PBH scenario and astrophysical ones that may explain spinning binaries in the mass gap, like hierarchical mergers [27][28][29][30][31][32] or others [33,34].…”

“…However, such a direct collapse process requires a very high stellar mass ( 200 M ) and a very low metallicity (typical of population III stars) [17][18][19][20][21][22][23][24][25][26]. Furthermore, it is not excluded that BHs with masses in the mass gap might have an astrophysical origin due to hierarchical coalescence of smaller BHs [27][28][29][30][31][32] or via direct collapse of a stellar merger between an evolved star and a main sequence companion [33,34]. Finally, BHs in the mass gap could be explained by beyond Standard Model physics that reduces the pair instability [35].…”

Bayesian Evidence for Both Astrophysical and Primordial Black Holes: Mapping the GWTC-2 Catalog to Third-Generation Detectors

“…Within the standard stellar formation scenario, it is difficult to accomodate massive events due to the Pulsational Pair Supernova Instability (PPSN) preventing the formation of binaries with masses above ≈ 45M , even though the precise location of such a cut-off is still uncertain [78][79][80][81][82][83][84][85][86][87][88][89][90][91]. One interesting possibility within the astrophysical sector is that massive events in the catalog are coming from second generation mergers in globular clusters or galactic nuclei [92][93][94][95][96][97][98]. Our findings show that introducing a PBH population of binaries in the inference naturally leads to the interpretation of those events as coming from primordial binaries.…”

Exploring Features in the Binary Black Hole Population

“…Peaks and lack of mergers: In simplest terms, the lack of observations in the chirp mass range 10-12M and four well-placed peaks can be explained with the first peak arising due to the pile-up of binary black holes because of a mass gap and following peaks due to hierarchical merger scenario (Miller & Hamilton 2002;Antonini & Rasio 2016;Rodriguez et al 2019;Doctor et al 2021;Gerosa & Fishbach 2021;Mapelli et al 2021). Fig.…”