Discovery of the most luminous quasar of the last 9 Gyr

Abstract

AbstractWe 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 Mgii, H$\unicode{x03B2}$, H$\unicode{x03B1}$, and Pa$\unicode{x03B2}$emission lines, from which we measure a black hole mass of$\log_{10}\! (M_{\mathrm{BH}}/\mathrm{M}_{\odot}) = 9.4 \pm 0.5$. With its high luminosity,$L_{\mathrm{bol}} = (4.7\pm1.0)\times10^{47}\,\mathrm{erg\,s}^{-1}$or$M_{i}(z=2) = -29.74$mag (AB), we estimate an Eddington ratio of$\approx1.4$. As the most luminous quasar known over the last${\sim}$9 Gyr of cosmic history, having a luminosity$8\times$greater than 3C 273, the source offers a range of potential follow-up opportunities.