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

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 $\sim 100$ 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.

Figure 1

The number of BBH inspirals per model at 12 Gyr for each of our 48 GC models as a function of final cluster mass. We show the weighted linear regression (with $1\sigma$ uncertainties on the slope) for the low and high metallicity models.

Figure 2

A sample distribution of inspirals in redshift from the set of models. The redshift is computed by assuming that the difference between the present day and the inspiral time corresponds to the cosmological lookback time at a given redshift (e.g., Ref. [42]). The number of inspirals drawn from each model is proportional to its weight, or how similar it is to the observed distribution of MWGCs. Inspirals of BBHs that were formed primordially are indicated with stars (merged in the cluster) and diamonds (ejected before merger). Inspirals of BBHs formed dynamically are shown as squares (in-cluster) and circles (ejected). Note that there are no binaries that are formed by binary stellar evolution with chirp masses greater than $\sim 13M_{\odot }$ (dashed line). This result is consistent across all models. The blue shaded regions illustrate the regions of parameter space where 50%, 10%, and 1% of sources are detectable by Advanced LIGO.