EVOLUTION OF SUPERMASSIVE BLACK HOLE BINARIES AND ACCELERATION OF JET PRECESSION IN GALACTIC NUCLEI

摘要

Supermassive black hole binaries (SMBHBs) are expected with the hierarchical galaxy formation model. Currently, physics processes dominating the evolution of a SMBHB are unclear. An interesting question is whether we could ob-servationally determine the evolution of SMBHBs and give constraints on the physical processes. Jet precession has been observed in many active galactic nuclei (AGNs) and is generally attributed to disk precession. In this paper we calculate the time variation of jet precession and conclude that jet precession is accelerated in SMBHB systems but decelerated in others. The acceleration of jet precession, dP_(pr)/dt, is related to the jet precession timescale, P_(pr), and the SMBHB evolution timescale, τ_a, as dP_(pr)/dt ≈ -A(P_(pr)/τ_a). Our calculations based on the models for jet precession and SMBHB evolution show that dP_(pr)/dt can be as high as about -1.0, with a typical value of -0.2, and can be easily detected. We discuss the differential jet precession for NGC 1275 that has been observed in the literature. If its observed rapid acceleration of jet precession is true, the jet precession is due to the orbital motion of an unbound SMBHB with a mass ratio of q ≈ 0.76. When jets precess from ancient bubbles to the currently active jets, the separation of the SMBHB decreases from about 1.46 kpc to 0.80 kpc, with an averaged decreasing velocity of da/dt ≈~-1.54 ×10~6 cm s~(-1) and an evolution timescale of τ_a ≈ 7.5 × 10~7 yr. However, if we assume steady jet precession formany cycles, the observations imply a hard SMBHB with a mass ratio of a q ≈ 0.21 and a separation of a ≈ 0.29 pc.