The environment near super massive black holes (SMBHs) in galactic nuclei contains a large number of stars and compact objects. A fraction of these are likely to be members of binaries. Here we discuss the binary population of stellar black holes and neutron stars near SMBHs and focus on the secular evolution of such binaries, due to the perturbation by the SMBH. Binaries with highly inclined orbits in respect to their orbit around the SMBH are strongly affected by secular Kozai processes, which periodically change their eccentricities and inclinations (Kozai-cycles). During periapsis approach, at the highest eccentricities during the Kozai-cycles, gravitational wave emission becomes highly efficient. Some binaries in this environment can inspiral and coalesce at timescales much shorter than a Hubble time and much shorter than similar binaries which do not reside near a SMBH. The close environment of SMBHs could therefore serve as catalyst for the inspiral and coalescence of binaries, and strongly affect their orbital properties. Such compact binaries would be detectable as gravitational wave (GW) sources by the next generation of GW detectors (e.g. advanced-LIGO). About 0.5 % of such nuclear merging binaries will enter the LIGO observational window while on orbit that are still very eccentric (e 0.5). The efficient gravitational wave analysis for such systems would therefore require the use of eccentric templates. We also find that binaries very close to the SMBH could evolve through a complex dynamical (non-secular) evolution leading to emission of several GW pulses during only a few yrs (though these are likely to be rare). Finally, we note that the formation of close stellar binaries, X-ray binaries and their merger products could be induced by similar secular processes, combined with tidal friction rather than GW emission as in the case of compact object binaries.

Motivated by the recent detection of gravitational waves from the black hole binary merger GW150914, we study the dynamical evolution of (stellar mass) black holes in galactic nuclei where massive star clusters reside. With masses of ∼ 10 7 M and sizes of only a few parsecs, nuclear star clusters are the densest stellar systems observed in the local universe and represent a robust environment where black hole binaries can dynamically form, harden and merge. We show that due to their large escape speeds, nuclear star clusters can retain a large fraction of their merger remnants. Successive mergers can then lead to significant growth and produce black hole mergers of several tens of solar masses similar to GW150914 and up to a few hundreds of solar masses, without the need of invoking extremely low metallicity environments. We use a semi-analytical approach to describe the dynamics of black holes in massive star clusters. Our models give a black hole binary merger rate of ≈ 1.5 Gpc −3 yr −1 from nuclear star clusters, implying up to a few tens of possible detections per year with Advanced LIGO. Moreover, we find a local merger rate of ∼ 1 Gpc −3 yr −1 for high mass black hole binaries similar to GW150914; a merger rate comparable to that of similar binaries assembled dynamically in globular clusters. Finally, we show that if all black holes receive high natal kicks, 50 km s −1 , then nuclear star clusters will dominate the local merger rate of binary black holes compared to either globular clusters or isolated binary evolution.

The second LIGO-Virgo catalog of gravitational-wave (GW) transients has more than quadrupled the observational sample of binary black holes. We analyze this catalog using a suite of five state-of-the-art binary black hole population models covering a range of isolated and dynamical formation channels and infer branching fractions between channels as well as constraints on uncertain physical processes that impact the observational properties of mergers. Given our set of formation models, we find significant differences between the branching fractions of the underlying and detectable populations, and that the diversity of detections suggests that multiple formation channels are at play. A mixture of channels is strongly preferred over any single channel dominating the detected population: an individual channel does not contribute to more than 70% of the observational sample of binary black holes. We calculate the preference between the natal spin assumptions and common-envelope efficiencies in our models, favoring natal spins of isolated black holes of 0.1, and marginally preferring common-envelope efficiencies of 2.0 while strongly disfavoring highly inefficient common envelopes. We show that it is essential to consider multiple channels when interpreting GW catalogs, as inference on branching fractions and physical prescriptions becomes biased when contributing formation scenarios are not considered or incorrect physical prescriptions are assumed. Although our quantitative results can be affected by uncertain assumptions in model predictions, our methodology is capable of including models with updated theoretical considerations and additional formation channels.

We consider the formation of binary black hole mergers through the evolution of field massive triple stars. In this scenario, favorable conditions for the inspiral of a black hole binary are initiated by its gravitational interaction with a distant companion, rather than by a common-envelope phase invoked in standard binary evolution models. We use a code that follows self-consistently the evolution of massive triple stars, combining the secular triple dynamics (Lidov-Kozai cycles) with stellar evolution. After a black hole triple is formed, its dynamical evolution is computed using either the orbit-averaged equations of motion, or a high-precision direct integrator for triples with weaker hierarchies for which the secular perturbation theory breaks down. Most black hole mergers in our models are produced in the latter non-secular dynamical regime. We derive the properties of the merging binaries and compute a black hole merger rate in the range (0.3 − 1.3) Gpc −3 yr −1 , or up to ≈ 2.5 Gpcif the black hole orbital planes have initially random orientation. Finally, we show that black hole mergers from the triple channel have significantly higher eccentricities than those formed through the evolution of massive binaries or in dense star clusters. Measured eccentricities could therefore be used to uniquely identify binary mergers formed through the evolution of triple stars. While our results suggest up to ≈ 10 detections per year with Advanced-LIGO, the high eccentricities could render the merging binaries harder to detect with planned space based interferometers such as LISA.