# Globular clusters as gravitational wave factories

TITLE: Dynamical Formation of the GW150914 Binary Black Hole
AUTHORS: Carl L. Rodriguez, Carl-Johan Haster, Sourav Chatterjee, Vicky Kalogera, Frederic A. Rasio
FIRST AUTHOR INSTITUTION: Center for Interdisciplinary Exploration and Research in Astrophysics, Northwestern University

The recent detection of the first gravitational wave signal (GW150914) produced by a binary black hole merger represents the dawn of a new age in astronomy. While the detection is significant in and of itself, one of the numerous questions yet to be answered is how such a binary black hole (BBH) system formed in the first place.

Previous models of BBH systems suggest that these are formed in the dense cores of globular clusters, where black holes left over from old, collapsed stars eventually become bound together through gravitational processes. After the most massive stars collapse, the resulting black holes tend to sink to the center of clusters due to their higher masses relative to other objects. This leads to a particularly high concentration of black holes near the cluster center, and also increases the rate of close gravitational encounters between these objects. BBH systems are continuously being formed and disrupted through complex interactions with other objects and systems in a globular cluster’s dense central environment, as illustrated in Fig. 1.

Fig. 1: Two possible formation histories for the GW150914 BBH system (indicated by the pair of orbiting black holes). Essentially, this is an example of the complex formation histories that BBH candidates go through within a globular cluster. Before two black holes become bound and merge together, they undergo numerous interactions with other objects (ex. stars and other black holes, indicated by the blue and red spheres).

The authors of today’s paper explore the conditions and environment required to form the system that lead to the detection of GW150914. To do this, the authors refer to their previous models of globular clusters that detail the distribution of stellar masses and number of binary systems. From these models, the authors select binary systems that have physical properties similar to that of GW150914 (ex. in mass and redshift). They find that most of these binary systems come from globular clusters of relatively low metallicities and large cluster masses. Lower metallicity stars tend to have weaker stellar winds, which means that they lose less mass over their lifetimes. These stars then tend to produce more massive black holes once they collapse at the end of their lifetimes. Additionally, more massive clusters tend to have a large number of black holes, which then increases the probability of a BBH system forming. The paper concludes that the globular cluster hosting the GW150914 progenitor system must have been a low metallicity cluster with a mass between $3 \times 10^5 and$6 \times 10^5\$ solar masses.

The masses of the BBH systems that could be detected by Advanced LIGO depends on the sensitivity of its detectors, and the distribution of these types of BBH systems in mass and redshift. Based on their models, the authors find that the median total mass of a detectable BBH during Advanced LIGO’s first observing run is 50 solar masses, which is consistent with the estimated mass of the GW150914 progenitor. While LIGO only has a single definitive BBH detection to its credit so far, future BBH merger detections from LIGO will help contrain the models and dynamical processes involves in these merger events.