Abstract
Supermassive black holes (SMBHs) may not always reside right at the centers of their host galaxies. This is a prediction of numerical relativity simulations, which imply that the newly formed single SMBH, after binary coalescence in a galaxy merger, can receive kick velocities up to several 1000 km/s due to anisotropic emission of gravitational waves. Long-lived oscillations of the SMBHs in galaxy cores, and in rare cases even SMBH ejections from their host galaxies, are the consequence. Observationally, accreting recoiling SMBHs would appear as quasars spatially and/or kinematically offset from their host galaxies. The presence of the “kicks” has a wide range of astrophysical implications which only now are beginning to be explored, including consequences for black hole and galaxy assembly at the epoch of structure formation, black hole feeding, and unified models of active galactic nuclei (AGN). Here, we review the observational signatures of recoiling SMBHs and the properties of the first candidates which have emerged, including follow-up studies of the candidate recoiling SMBH of SDSSJ092712.65+294344.0.
Highlights
Interaction and merging of galaxies occurs frequently throughout the history of the universe
Galaxy mergers are believed to be the sites of major black hole growth, and an active search for SMBH pairs and binaries of wide and small separations is currently ongoing
Kick velocity was highest for maximally spinning equal-mass black hole binaries with antialigned spins in the orbital plane (“superkicks”)
Summary
Interaction and merging of galaxies occurs frequently throughout the history of the universe. Configurations of coalescing black holes can lead to kick velocities up to several thousand km/s (e.g., [4,5,6,7,8,9,10,11,12,13,14,15,16]; review by [17]). In a “typical,” gas-poor galaxy, a black hole kick velocity of 500 km/s will result in an initial amplitude of ∼200 pc, and an oscillation timescale of order 107 yrs (Figure 1 of [23]). The kicks, including those large enough to remove SMBHs from their host galaxies, have potentially far-reaching astrophysical consequences, including for SMBH and galaxy assembly and AGN statistics.
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