SDSS J0909$+$4449: A Large-separation Strongly Lensed Quasar at $z \sim 2.8$ with Three Images

Abstract: We report the discovery of SDSS J0909$+$4449, an exceptional system consisting of a quasar at $z=2.788$ strongly lensed by a group of galaxies at $z \sim 0.9$ into three images separated by up to 14\arcsec\, based on archival data collected by the Sloan Digital Sky Survey, extended Baryon Oscillation Spectroscopic Survey, Beijing--Arizona Sky Survey, the Mayall z-band Legacy Survey, and the Gemini Telescope. We discuss two hypotheses on the nature of SDSS J0909$+$4449, i.e. a rare triply-imaged quasar in the naked cusp configuration and a typical quadruply-imaged quasar with the fourth image undetected in the current data. We find that simple lens models can provide excellent fits to the observed image positions and the non-detection under either hypothesis. Deeper imaging data, spectroscopic observations, and follow-up light curve measurements will be helpful in determining which hypothesis is correct and provide better constraints on the lens mass distribution. Nevertheless, given its unusually large image separations, SDSS J0909$+$4449 will be a unique probe for the mass structure and the underlying cooling and stellar feedback processes on group or cluster scales.