v 1 6 J an 2 00 7 Discovery of a Probable Physical Triple Quasar 1

We report the discovery of the first known probable case of a physical triple quasar (not a gravitational lens). A previously known double system, QQ 1429− 008 at z = 2.076, is shown to contain a third, fainter QSO component at the same redshift within the measurement errors. Deep optical and IR imaging at the Keck and VLT telescopes has failed to reveal a plausible lensing galaxy group or a cluster, and moreover, we are unable to construct any viable lensing model which could lead to the observed distribution of source positions and relative intensities of the three QSO image components. Furthermore, there are hints of differences in broad-band spectral energy distributions of different components, which are more naturally understood if they are physically distinct AGN. Therefore, we conclude that this system is most likely a physical triple quasar, the first such close QSO grouping known at any redshift. The projected component separations in the restframe are ∼ 30 − 50 kpc for the standard concordance cosmology, typical of interacting galaxy systems. The existence of this highly unusual system supports the standard picture in which galaxy interactions lead to the onset of QSO activity. Subject headings: gravitational lensing — galaxies: interactions — quasars: general Based in part on the data obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and NASA, made possible by the generous financial support of the W.M. Keck Foundation; and data obtained with VLT/ANTU telescope at ESO-Paranal Observatory; and data obtained with the Hubble Space Telescope, operated by NASA. This research also made use of data obtained from the US National Virtual Observatory, which is sponsored by the National Science Foundation. Division of Physics, Mathematics, and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA. Laboratoire d’Astrophysique, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1290 Sauverny, Switzerland. Large Binocular Telecope Observatory, Univ. of Arizona, Tucson, AZ 85721, USA.