Scientific Seminars
Time evolution of the spins of supermassive black holes pairs
Albino Perego
Universita' di Milano
2008-05-27 15.00 Bicocca -
During the last years several independent observations have conrmed conrm the presence of supermassive black holes (SMBH, 10^6M_sun < MSMBH < 10^9M_sun), in the center of massive galaxies. When two of these gas-rich galaxies collide, trough gravitational interaction, to form a single galaxy (gas rich merger), a great amount of gas falls in the center of the system and forms a very massive, self-gravitating, circumnuclear disc. The two SMBH are dragged, together with the gas, into the circumnuclear disc, and start to rotate and to fall toward the center, as an eect of dynamical friction: on a timescale of 10^7 yr the two compact objects form a keplerian binary and, after the emission of GW, they coalesce to become a single SMBH (SMBH remnant). If the spins of the two SMBH were misaligned and misaligned with the orbital angular momentum of the binary, the powerful, anisotropical emission of GW could eject the remnant from the new host galaxy. A possible way to prevent this situation is the alignment of both the SMBH spins with the angular momentum of the circumnuclear disc, before binary formation: this process of alignment could be the result of a relativistic spin-orbit coupling, known as Lense-Thirring effect. On a small timescale (10^3 yr) Lense-Thirring effect causes a misaligned accretion disc to aligned the orbital angular momentum of its inner part with the spin of the SMBH (Bardeen-Petterson effect); on a longer timescale, whose value is determined through a simple analitical model, the disc exerts a torque on the hole which modify significantly its spin. |