Stellar-mass black holes in young massive and open stellar clusters and their role in gravitational-wave generation

Abstract

Stellar-remnant black holes (BH) in dense stellar clusters have always drawn attention due to their potential in a number of phenomena, especially the dynamical formation of binary black holes (BBH), which potentially coalesce via gravitational-wave (GW) radiation. This study presents a preliminary set of evolutionary models of compact stellar clusters with initial masses ranging over 1.0 × 10 4 M − 5.0 × 10 4 M , and halfmass radius of 2 or 1 pc, that is typical for young massive and starburst clusters. They have metallicities between 0.05Z − Z . Including contemporary schemes for stellar wind and remnant formation, such model clusters are evolved, for the first time, using the state-of-the-art direct N-body evolution program NBODY7 , until their dissolution or at least for 10 Gyr. That way, a self-regulatory behaviour in the effects of dynamical interactions among the BHs is demonstrated. In contrast to earlier studies, the BBH coalescences obtained in these models show a prominence in triple-mediated coalescences while being bound to the clusters, compared to those occurring among the BBHs that are dynamically ejected from the clusters. A broader mass spectrum of the BHs and lower escape velocities of the clusters explored here might cause this difference, which is yet to be fully understood. Among the BBH coalescences obtained here, there are ones that resemble the detected GW151226, LVT151012, and GW150914 events and also ones which are even more massive. A preliminary estimate suggests few 10s-100s of BBH coalescences per year, originating due to dynamics in stellar clusters, that can be detected by the LIGO at its design sensitivity.

Stellar black hole binary mergers in open clusters

“…The most updated estimate of the merger rate from LIGO-Virgo events (after including GW170104) is 12-213 Gpc −3 yr −1 (Abbott et al 2017a). Our BHB merger rate is consistent with those found in Banerjee (2017Banerjee ( , 2018b for BHB mergers in Young Massive Star Clusters (YMSC). found a merger rate ranging from 0.05 to 1 Gpc −3 yr −1 for possible progenitor of GW150914 in globular clusters at z<0.3.…”

Constraining the Fraction of Binary Black Holes Formed in Isolation and Young Star Clusters with Gravitational-wave Data

“…Lada & Lada 2003;Portegies Zwart et al 2010). However, only few studies focus on BBH formation in young star clusters, because this is a computational challenge (Ziosi et al 2014;Mapelli 2016;Banerjee 2017Banerjee , 2018Kumamoto et al 2019;Di Carlo et al 2019). Here, we consider the largest sample of merging BBHs produced in a set of N-body simulations of young star clusters (Di Carlo et al 2019).…”

“…Antonini & Perets 2012;Antonini & Rasio 2016;Kimpson et al 2016;Antonini et al 2017). Similarly, dynamical exchanges and three-or multi-body scatterings are expected to lead to the formation and dynamical hardening of BBHs in dense stellar systems, such as globular clusters (Portegies Zwart & McMillan 2000;O'Leary et al 2006;Sadowski et al 2008;Downing et al 2010Downing et al , 2011Rodriguez et al 2015Rodriguez et al , 2016aRodriguez & Loeb 2018;Askar et al 2017;Samsing 2018;Samsing et al 2018;Fragione & Kocsis 2018), nuclear star clusters (O'Leary et al 2009;Antonini & Perets 2012;Antonini & Rasio 2016;Petrovich & Antonini 2017;Stone et al 2017b,a;Rasskazov & Kocsis 2019) and young star clusters (Banerjee et al 2010;Mapelli et al 2013;Ziosi et al 2014;Mapelli 2016;Banerjee 2017Banerjee , 2018Di Carlo et al 2019;Kumamoto et al 2019). Other formation mechanisms include black hole (BH) pairing in extreme gaseous environments (like AGN disks, e.g.…”

On the merger rate of primordial black holes: effects of nearest neighbours distribution and clustering

“…For instance, a dynamical origin of the binaries, notably in young massive and open stellar clusters, is a viable (albeit uncertain) possibility, see e.g. [18,19]. Furthermore, a multi-channel origin, in particular the idea that some or all of the mergers could be primordial -a proposal dating back to [20]; see [21] for a recent review-cannot be disregarded at the moment [17].Soon after the first merger was detected, some authors [22,23] went as far as conjecturing that the population of BHs responsible for these mergers is, in fact, not only primordial, but also accounts for the still unidentified dark matter (DM) which constitutes about one quarter of the energy density of the Universe [24].…”