Discovery of the most luminous quasar of the last 9 Gyr
Résumé
We report the discovery of a bright ( $g = 14.5$ mag (AB), $K = 11.9$ mag (Vega)) quasar at redshift $z=0.83$ — the optically brightest (unbeamed) quasar at $z>0.4$ . SMSS J114447.77-430859.3, at a Galactic latitude of $b=+18.1^{\circ}$ , was identified by its optical colours from the SkyMapper Southern Survey (SMSS) during a search for symbiotic binary stars. Optical and near-infrared spectroscopy reveals broad Mg II, H $\unicode{x03B2}$ , H $\unicode{x03B1}$ , and Pa $\unicode{x03B2}$ emission lines, from which we measure a black hole mass of $\log_{10}\! (M_{BH}/M_{\odot}) = 9.4 ± 0.5$ . With its high luminosity, $L_{bol} = (4.7±1.0)×10^{47} erg s^{-1}$ or $Mi(z=2) = -29.74$ mag (AB), we estimate an Eddington ratio of $≈1.4$ . As the most luminous quasar known over the last ${∼}$ 9 Gyr of cosmic history, having a luminosity $8×$ greater than 3C 273, the source offers a range of potential follow-up opportunities.