Possible evidence for the ejection of a supermassive black hole from an ongoing merger of galaxies

Attempts of Magain et al. to detect the host galaxy of the bright quasi-stellar object (QSO) HE0450-2958 have not been successful. We suggest that the supermassive black hole (SMBH) powering the QSO was ejected from the observed ultraluminous infrared galaxy (ULIRG) at the same redshift and at 1.5 arcsec distance. Ejection could have been caused either by recoil due to gravitational wave emission from a coalescing binary of SMBHs or the gravitational slingshot of three or more SMBHs in the ongoing merger of galaxies which triggered the starburst activity in the ULIRG. We discuss implications for the possible hierarchical build-up of SMBHs from intermediate and/or stellar mass black holes, and for the detection of coalescing supermassive binary black holes by LISA.