Binary Black Hole Mergers from Globular Clusters: Implications for Advanced LIGO

Abstract

The predicted rate of binary black hole mergers from galactic fields can vary over several orders of magnitude and is extremely sensitive to the assumptions of stellar evolution. But in dense stellar environments such as globular clusters, binary black holes form by well-understood gravitational interactions. In this letter, we study the formation of black hole binaries in an extensive collection of realistic globular cluster models. By comparing these models to observed Milky Way and extragalactic globular clusters, we find that the mergers of dynamically-formed binaries could be detected at a rate of ∼ 100 per year, potentially dominating the binary black hole merger rate. We also find that a majority of cluster-formed binaries are more massive than their field-formed counterparts, suggesting that Advanced LIGO could identify certain binaries as originating from dense stellar environments.

Binary Black Hole Mergers in the First Advanced LIGO Observing Run

“…For example, if all merging BBHs arise from dynamical formation in globular clusters, then the lower limit on the merger rate disfavors low-mass clusters [165]. On the other hand, if all merging BBHs arise from isolated binaries evolving via the common-envelope phase, the lower limit on the merger rate disfavors a combination of very-low common envelope binding energy with a high efficiency of common envelope ejection [189] (high values of α × λ, as defined in Refs.…”

“…Possible BBH formation channels include dynamical formation in a dense stellar environment (see, e.g., Refs. [161][162][163][164][165]), possibly assisted by gas drag in galactic nuclear disks [166,167], or isolated binary evolution, either the classical variant via a common-envelope phase (see, e.g., Refs. [168][169][170][171][172][173]), possibly from population III binaries [174,175], or chemically homogeneous evolution in close tidally locked binaries [176,177].…”

Complete waveform model for compact binaries on eccentric orbits

“…In these environments, compact systems can undergo a variety of N-body interactions that lead to the formation of compact binaries that retain eccentricity during their lifetime (see Refs. [7,10,[17][18][19][20] and references therein).…”

Distinguishing spin-aligned and isotropic black hole populations with gravitational waves

“…One signature of such environments is the angular distribution of the black hole spins. Binary systems that formed through dynamical interactions between alreadycompact objects are expected to have isotropic spin orientations [5][6][7][8][9] (that is, the spins of the black holes are randomly oriented with respect to the orbit of the binary system), whereas those that formed from pairs of stars born together are more likely to have spins that are preferentially aligned with the orbit [10][11][12][13][14] . The bestmeasured combination of spin parameters 3,4 for each of the four likely binary black hole detections GW150914, LVT151012, GW151226 and GW170104 is the 'effective' spin.…”

“…The spin directions of binary black holes that are formed dynamically through interactions in dense stellar environments [5][6][7][8] are expected to be isotropic, owing to the absence of a preferred direction 9 and the persistence of an isotropic distribution throughout in-spiralling 29,30 . Online Content Methods, along with any additional Extended Data display items and Source Data, are available in the online version of the paper; references unique to these sections appear only in the online paper.…”