Discovery of the most ultra-luminous QSO using Gaia, SkyMapper and WISE

TL;DR

This paper reports the discovery of the most ultra-luminous, unlensed quasar at high redshift, identified through a combination of Gaia, SkyMapper, and WISE data, expanding our understanding of the brightest quasars in the universe.

Contribution

The paper introduces a novel method combining Gaia proper-motion data with SkyMapper and WISE photometry to discover the most luminous unlensed quasar known.

Findings

Identified a quasar with the highest known UV-optical luminosity.

The quasar is unlikely to be strongly gravitationally lensed or beamed.

It exhibits weak emission lines and broad absorption features.

Abstract

We report the discovery of the ultra-luminous QSO SMSS~J215728.21-360215.1 with magnitude $z=16.9$ and W4$=7.42$ at redshift 4.75. Given absolute magnitudes of $M_{145,\rm AB}=-29.3$, $M_{300,\rm AB}=-30.12$ and $\log L_{\rm bol}/L_{\rm bol,\odot} = 14.84$, it is the QSO with the highest unlensed UV-optical luminosity currently known in the Universe. It was found by combining proper-motion data from Gaia DR2 with photometry from SkyMapper DR1 and the Wide-field Infrared Survey Explorer (WISE). In the Gaia database it is an isolated single source and thus unlikely to be strongly gravitationally lensed. It is also unlikely to be a beamed source as it is not discovered in the radio domain by either NVSS or SUMSS. It is classed as a weak-emission-line QSO and possesses broad absorption line features. A lightcurve from ATLAS spanning the time from October 2015 to December 2017 shows little sign of variability.