Galaxies with stellar bulges are generically observed to host supermassive black holes (SMBHs). The hierarchical merging of galaxies should therefore lead to the formation of SMBH binaries. Merging of old massive galaxies with little gas promotes the formation of low‐density nuclei where SMBH binaries are expected to survive over long times. If the binary lifetime exceeds the typical time between mergers, then triple black hole (BH) systems may form. We study the statistics of close triple SMBH encounters in galactic nuclei by computing a series of three‐body orbits with physically motivated initial conditions appropriate for giant elliptical galaxies. Our simulations include a smooth background potential consisting of a stellar bulge plus a dark matter halo, drag forces due to gravitational radiation and dynamical friction on the stars and dark matter, and a simple model of the time evolution of the inner density profile under heating and mass ejection by the SMBHs. We find that the binary pair coalesces as a result of repeated close encounters in ∼85 per cent of our runs, and in ∼15 per cent of cases a new eccentric binary forms from the third SMBH and binary remnant and coalesces during the run time. In about 40 per cent of the runs the lightest BH is left wandering through the galactic halo or escapes the galaxy altogether, but escape of all three SMBHs is exceedingly rare. The triple systems typically scour out cores with mass deficits ∼one–two times their total mass, which can help to account for the large cores observed in some massive elliptical galaxies, such as M87. The high coalescence rate, prevalence of very high‐eccentricity orbits, and gravitational radiation ‘spikes’ during close encounters in our runs, may provide interesting signals for the future Laser Interferometer Space Antenna (LISA).

Since the violent relaxation in hierarchical merging is incomplete, elliptical galaxies retain a wealth of information about their formation pathways in their present-day orbital structure. Recent advances in integral field spectroscopy, multi-slit infrared spectroscopy, and triaxial dynamical modeling techniques have greatly improved our ability to harvest this information. A variety of observational and theoretical evidence indicates that gas-rich major mergers play an important role in the formation of elliptical galaxies. We simulate 1:1 disk mergers at seven different initial gas fractions (f gas ) ranging from 0 to 40%, using a version of the TreeSPH code Gadget-2 that includes radiative heating and cooling, star formation, and feedback from supernovae and active galactic nuclei. We classify the stellar orbits in each remnant and construct radial profiles of the orbital content, intrinsic shape, and orientation. The dissipationless remnants are typically prolate-triaxial, dominated by box orbits within r c ∼ 1.5R e , and by tube orbits in their outer parts. As f gas increases, the box orbits within r c are increasingly replaced by a population of short axis tubes (z−tubes) with near zero net rotation, and the remnants become progressively more oblate and round. The long axis tube (x−tube) orbits are highly streaming and relatively insensitive to f gas , implying that their angular momentum is retained from the dynamically cold initial conditions. Outside r c , the orbital structure is essentially unchanged by the gas. For f gas 15%, gas that retains its angular momentum during the merger re-forms a disk, that appears in the remnants as a highly streaming z−tube population superimposed on the hot z−tube distribution formed by the old stars. In the 15-20% gas remnants, this population appears as a kinematically distinct core (KDC) within a system that is slowly rotating or dominated by minor-axis rotation. These remnants show an interesting resemblance, in both their velocity maps and intrinsic orbital structure, to the KDC galaxy NGC4365 (van den Bosch et al. 2008). At 30-40% gas, the remnants are rapidly rotating, with sharp embedded disks on ∼ 1R e scales. We predict a characteristic, physically intuitive orbital structure for 1:1 disk merger remnants, with a distinct transition between 1 and 3R e that will be readily observable with combined data from the 2D kinematics surveys SAURON and SMEAGOL. Our results illustrate the power of direct comparisons between N −body simulations and dynamical models of observed systems to constrain theories of galaxy formation.

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