Recent numerical relativistic simulations of black hole coalescence suggest that in certain alignments the emission of gravitational radiation can produce a kick of several thousand kilometers per second. This exceeds galactic escape speeds, hence unless there a mechanism to prevent this, one would expect many galaxies that had merged to be without a central black hole. Here we show that in most galactic mergers, torques from accreting gas suffice to align the orbit and spins of both black holes with the large-scale gas flow. Such a configuration has a maximum kick speed < 200 km s −1 , safely below galactic escape speeds. We predict, however, that in mergers of galaxies without much gas, the remnant will be kicked out several percent of the time. We also discuss other predictions of our scenario, including implications for jet alignment angles and X-type radio sources.

Coincident detections of electromagnetic (EM) and gravitational wave (GW) signatures from coalescence events of supermassive black holes are the next observational grand challenge. Such detections will provide the means to study cosmological evolution and accretion processes associated with these gargantuan compact objects. More generally, the observations will enable testing general relativity in the strong, nonlinear regime and will provide independent cosmological measurements to high precision. Understanding the conditions under which coincidences of EM and GW signatures arise during supermassive black hole mergers is therefore of paramount importance. As an essential step towards this goal, we present results from the first fully general relativistic, hydrodynamical study of the late inspiral and merger of equal-mass, spinning supermassive black hole binaries in a gas cloud. We find that variable EM signatures correlated with GWs can arise in merging systems as a consequence of shocks and accretion combined with the effect of relativistic beaming. The most striking EM variability is observed for systems where spins are aligned with the orbital axis and where orbiting black holes form a stable set of density wakes, but all systems exhibit some characteristic signatures that can be utilized in searches for EM counterparts. In the case of the most massive binaries observable by the Laser Interferometer Space Antenna, calculated luminosities imply that they may be identified by EM searches to z ≈ 1, while lower mass systems and binaries immersed in low density ambient gas can only be detected in the local universe.

The intracluster medium (ICM) of galaxy clusters is a weakly collisional, high-beta plasma in which the transport of heat and momentum occurs primarily along magnetic-field lines. Anisotropic heat conduction allows convective instabilities to be driven by temperature gradients of either sign, the magnetothermal instability (MTI) in the outskirts of non-isothermal clusters and the heat-flux buoyancy-driven instability (HBI) in their cooling cores. We employ the Athena magnetohydrodynamic code to investigate the nonlinear evolution of these instabilities, self-consistently including the effects of anisotropic viscosity (i.e. Braginskii pressure anisotropy), anisotropic conduction, and radiative cooling. We highlight the importance of the microscale instabilities (firehose, mirror) that inevitably accompany and regulate the pressure anisotropies generated by the HBI and MTI. We find that, in all but the innermost regions of cool-core clusters, anisotropic viscosity significantly impairs the ability of the HBI to reorient magnetic-field lines orthogonal to the temperature gradient. Thus, while radio-mode feedback appears necessary in the central few tens of kpc, heat conduction may be capable of offsetting radiative losses throughout most of a cool core over a significant fraction of the Hubble time. Magnetically-aligned cold filaments are then able to form by local thermal instability. Viscous dissipation during the formation of a cold filament produces accompanying hot filaments, which can be searched for in deep Chandra observations of nearby cool-core clusters. In the case of the MTI, anisotropic viscosity maintains the coherence of magnetic-field lines over larger distances than in the inviscid case, thereby providing a natural lower limit for the scale on which the field can fluctuate freely. In the nonlinear state, the magnetic field exhibits a folded structure in which the field-line curvature and field strength are anti-correlated. These results demonstrate that, if the HBI and MTI are relevant for shaping the properties of the ICM, one must self-consistently include anisotropic viscosity in order to obtain even qualitatively correct results.

A search for recoiling supermassive black hole candidates recently yielded the best candidate thus far, SDSS J092712.65+294344.0 reported by Komossa et al. Here we propose the alternative hypothesis that this object is a supermassive black hole binary. From the velocity shift imprinted in the emission-line spectrum we infer an orbital period of ∼ 190 years for a binary mass ratio of 0.1, a secondary black hole mass of 10 8 M ⊙ , and assuming inclination and orbital phase angles of 45 • . In this model the origin of the blueshifted narrow emission lines is naturally explained in the context of an accretion flow within the inner rim of the circumbinary disk. We attribute the blueshifted broad emission lines to gas associated with a disk around the accreting secondary black hole. We show that, within the uncertainties, this binary system can be long lived and thus, is not observed in a special moment in time. The orbital motion of the binary can potentially be observed with the V LBA if at least the secondary black hole is a radio emitter. In addition, for the parameters quoted above, the orbital motion will result in a ∼ 100 km s −1 velocity shift of the emission lines on a time scale of about a year, providing a direct observational test for the binary hypothesis.

We have been spectroscopically monitoring 88 quasars selected to have broad Hβ emission lines offset from their systemic redshift by thousands of km s −1 . By analogy with single-lined spectroscopic binary stars, we consider these quasars to be candidates for hosting supermassive black hole binaries (SBHBs). In this work we present new radial velocity measurements, typically 3-4 per object over a time period of up to 12 years in the observer's frame. In 29/88 of the SBHB candidates no variability of the shape of the broad Hβ profile is observed, which allows us to make reliable measurements of radial velocity changes. Among these, we identify three objects that have displayed systematic and monotonic velocity changes by several hundred km s −1 and are prime targets for further monitoring. Because the periods of the hypothetical binaries are expected to be long, we cannot hope to observe many orbital cycles during our lifetimes. Instead, we seek to evaluate the credentials of the SBHB candidates by attempting to rule out the SBHB hypothesis. In this spirit, we present a method for placing a lower limit on the period, and thus the mass, of the SBHBs under the assumption that the velocity changes we observe are due to orbital motion. Given the duration of our monitoring campaign and the uncertainties in the radial velocities, we were able to place a lower limit on the total mass in the range 4.7 × 10 4 − 3.8 × 10 8 M ⊙ , which does not yet allow us to rule out the SBHB hypothesis for any candidates.

The quest for binary and dual supermassive black holes (SMBHs) at the dawn of the multi-messenger era is compelling. Detecting dual active galactic nuclei (AGN)-active SMBHs at projected separations larger than several parsecs-and binary AGN-probing the scale where SMBHs are bound in a Keplerian binary-is an observational challenge. The study of AGN pairs (either dual or binary) also represents an overarching theoretical problem in cosmology and astrophysics. The AGN triggering calls for detailed knowledge of the hydrodynamical conditions of gas in the imminent surroundings of the SMBHs and, at the same time, their duality calls for detailed knowledge on how galaxies assemble through major and minor mergers and grow fed by matter along the filaments of the cosmic web. This review describes the techniques used across the electromagnetic spectrum to detect dual and binary AGN candidates and proposes new avenues for their search. The current observational status is compared with the state-of-the-art numerical simulations and models for formation of dual and binary AGN. Binary SMBHs are among the loudest sources of gravitational waves (GWs) in the Universe. The search for a background of GWs at nHz frequencies from inspiralling SMBHs at low redshifts, and the direct detection of signals from their coalescence by the Laser Interferometer Space Antenna in the next decade, make this a theme of major interest for multi-messenger astrophysics. This review discusses the future facilities and observational strategies that are likely to significantly advance this fascinating field.

We perform a suite of simulations of cooling cores in clusters of galaxies in order to investigate the effect of the recently discovered heat flux buoyancy instability (HBI) on the evolution of cores. Our models follow the 3-dimensional magnetohydrodynamics (MHD) of cooling cluster cores and capture the effects of anisotropic heat conduction along the lines of magnetic field, but do not account for the cosmological setting of clusters or the presence of AGN. Our model clusters can be divided into three groups according to their final thermodynamical state: catastrophically collapsing cores, isothermal cores, and an intermediate group whose final state is determined by the initial configuration of magnetic field. Modeled cores that are reminiscent of real cluster cores show evolution towards thermal collapse on a time scale which is prolonged by a factor of ∼ 2 − 10 compared with the zeroconduction cases. The principal effect of the HBI is to re-orient field lines to be perpendicular to the temperature gradient. Once the field has been wrapped up onto spherical surfaces surrounding the core, the core is insulated from further conductive heating (with the effective thermal conduction suppressed to less than 10 −2 of the Spitzer value) and proceeds to collapse. We speculate that, in real clusters, the central AGN and possibly mergers play the role of "stirrers," periodically disrupting the azimuthal field structure and allowing thermal conduction to sporadically heat the core.

We model the electromagnetic signatures of massive black hole binaries (MBHBs) with an associated gas component. The method comprises numerical simulations of relativistic binaries and gas coupled with calculations of the physical properties of the emitting gas. We calculate the UV/ X-ray and the H light curves and the H emission profiles. The simulations are carried out with a modified version of the parallel tree SPH code Gadget. The heating, cooling, and radiative processes are calculated for two different physical scenarios, where the gas is approximated as a blackbody or a solar metallicity gas. The calculation for the solar metallicity scenario is carried out with the photoionization code Cloudy. We focus on subparsec binaries that have not yet entered the gravitational radiation phase. The results from the first set of calculations, carried out for a coplanar binary and gas disk, suggest that there are pronounced outbursts in the X-ray light curve during pericentric passages. If such outbursts persist for a large fraction of the lifetime of the system, they can serve as an indicator of this type of binary. The predicted H emission line profiles may be used as a criterion for selection of MBHB candidates from existing archival data. The orbital period and mass ratio of a binary may be inferred after carefully monitoring the evolution of the H profiles of the candidates. The discovery of subparsec binaries is an important step in understanding of the merger rates of MBHBs and their evolution toward the detectable gravitational wave window.