Abstract: AGNs and all massive galaxies are believed to contain supermassive black holes at centre. Because galaxies grow through mergers, the central black holes must come together to form supermassive binaries (SMBHBs) after two galaxies merge. Measurements of SMBHBs can provide us with a statistical measure of how many galaxies are the results of a merger, and thus help to understand the evolution of the universe. In addition, SMBHBs are destined to merge in a burst of strong gravitational wave radiations: those that still elude detection and for which scientists are building more and more sensitive telescopes. A couple of SMBHBs have been directly detected in AGNs. Until recently, however, clear evidence of an example of a close SMB has been lacking. In particular, most binaries may form at gas-poor galactic nuclei but are extremely challenging to find them because they are truly dark. In this colloquium, I will talk about our long-lasting theoretical investigations about how to discover and measure SMBHBs with stellar tidal disruptions and show the discovery of the first pair of SMBHB in normal galaxies. At the end of my talk, I will discuss how many similar systems we expect to detect in the near future.

Biog: Dr. Fukun Liu is a full professor of astrophysics and the director of the Department of Astronomy, Peking University. He got his PhD in astrophysics at SISSA (Italy) under the supervision of Prof. John Miller, Dr. Antonio Lanza, and Prof. Dennis Sciama in 1999. He went to Chalmers University of Technology and Gothenburg University as a post-doctoral fellow of the Natural Science Research Foundation of Sweden (NFR fellow) from 1999 to 2001. He then joined Peking University first as a faculty member. His main research interests are supermassive black holes (SMBHs) and binaries, accretion processes around black holes, AGNs, electro-magnetic counterparts of gravitational wave radiations, transient activities of galactic nuclei, and stellar dynamics around SMBHs. Recently, he with his research group discovered the first pair of SMBHs in normal galaxies.