Dynamical Formation of Low-mass Merging Black Hole Binaries like GW151226

Abstract

Using numerical models for star clusters spanning a wide range in ages and metallicities (Z) we study the masses of binary black holes (BBHs) produced dynamically and merging in the local universe (z0.2). After taking into account cosmological constraints on star formation rate and metallicity evolution, which realistically relate merger delay times obtained from models with merger redshifts, we show here for the first time that while old, metal-poor globular clusters can naturally produce merging BBHs with heavier components, as observed in GW150914, lower-mass BBHs like GW151226 are easily formed dynamically in younger, higher-metallicity clusters. More specifically, we show that the mass of GW151226 is well within 1σ of the mass distribution obtained from our models for clusters with Z/Z e 0.5. Indeed, dynamical formation of a system like GW151226 likely requires a cluster that is younger and has a higher metallicity than typical Galactic globular clusters. The LVT151012 system, if real, could have been created in any cluster with Z/Z e 0.25. On the other hand, GW150914 is more massive (beyond 1σ) than typical BBHs from even the lowest-metallicity (Z/Z e =0.005) clusters we consider, but is within 2σ of the intrinsic mass distribution from our cluster models with Z/Z e 0.05; of course, detection biases also push the observed distributions toward higher masses.

LIGO and the opening of a unique observational window on the universe

“…The lower-mass systems, GW151226 and LVT151012, need the full range of ages and metallicities present among dense star clusters (44,78). On the other hand, the chemically homogeneous channel forms only massive systems and so is inconsistent with the two lighter mergers that were observed, but is consistent with GW150914.…”

“…As the detected sample grows, it will be possible to extract the underlying BH mass distribution, correcting for GW observational biases. The high masses already detected point to the need for relativity weak stellar winds and therefore formation environments with subsolar metallicities (43,44).…”

Dissipative Evolution of Unequal-mass Binary–single Interactions and Its Relevance to Gravitational-wave Detections

“…This is not a unique problem related to our model, as nearly all proposed BBH merger channels are plagued by the same large uncertainties related to the dynamics and BH demographics of GCs (e.g. Chatterjee et al 2017). …”

Spin–Orbit Misalignment of Merging Black Hole Binaries with Tertiary Companions

“…These small values could be due to the slow rotation of the BHs (Zaldarriaga et al 2017) or spinorbit misalignments. The latter possibility would imply a dynamical formation channel of the BH binaries (such as exchange interaction in globular clusters (Rodriguez et al 2015;Chatterjee et al 2017)) or, as our calculations indicate, dynamical influences of external companions.…”

“…The recent breakthrough in the detection of gravitational waves (GWs) from merging black hole (BH) binaries by advanced LIGO (Abbott et al 2016a(Abbott et al ,b, 2017 has generated renewed interest in understanding the formation mechanisms of compact BH binaries, from the evolution of massive stellar binaries (Lipunov et al 1997;Belczynski et al 2010Belczynski et al , 2016Mandel & de Mink 2016;Lipunov et al 2017) and triples (Silsbee & Tremaine 2017;Antonini et al 2017) in the galactic fields, to dynamical interactions in galactic nuclei (Antonini & Perets 2012;Petrovich & Antonini 2017) and in the dense core of globular clusters (Miller & Hamilton 2002;Rodriguez et al 2015;Chatterjee et al 2017).…”