Discovery of the most luminous quasar of the last 9 Gyr

Abstract

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◦, 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, Hβ, Hα, and Paβ emission lines, from which we measure a black hole mass of log10(MBH/M) = 9.4 ± 0.5. With its high luminosity, Lbol = (4.7 ± 1.0) × 1047 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.