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

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.

• The paper presents the discovery of SMSS J215728.21-360215.1, the most luminous unlensed QSO at z = 4.75, advancing our understanding of early universe phenomena.
• It employs Gaia DR2’s precise proper motion data alongside SkyMapper and WISE observations to effectively eliminate stellar contamination and accurately identify QSO candidates.
• The observed luminosity implies a supermassive black hole of around 20 billion solar masses, challenging existing theories of rapid black hole growth in the early cosmos.

Analysis of the Discovery of the Ultra-Luminous High-Redshift QSO

The paper by Wolf et al. presents a significant discovery in the study of quasars (QSOs), reporting the identification of SMSS J215728.21-360215.1, a high-redshift, ultra-luminous QSO. Utilizing data from multiple astronomical surveys, this research delineates the methods and implications of finding the most luminous unlensed QSO known in the UV-optical spectrum at the time of publication.

Methodological Advances

The discovery was facilitated by an integration of data from the European Space Agency's Gaia Data Release 2 (DR2), SkyMapper, and the Wide-field Infrared Survey Explorer (WISE). Key to this achievement was Gaia's ability to precisely measure proper motions, a capability that effectively eliminates the traditional contamination by stars that often plagues high-redshift QSO surveys. The candidate QSO was selected based on its apparent red colors and negligible proper motion, leading to a more efficient and targeted follow-up with other data sources.

Spectroscopic and Photometric Attributes

SMSS J215728.21-360215.1 was identified at a redshift of $z = 4.75$, based on weak emissions detected in its spectrum and strong broad absorption lines. The noted attributes include an absolute magnitude of $M_{300,\mathrm{AB}} = -30.12$ and a bolometric luminosity of approximately $6.95 \times 10^{14}$ times solar luminosity. Notably, the absence of significant radio emissions suggests that its optical brightness is unlikely to be enhanced by a relativistic jet, and as such, its intrinsic optical luminosity is unprecedented for an unlensed QSO.

Implications for Black Hole Formation and Cosmology

The observed luminosity implies the presence of a supermassive black hole with an estimated mass of 20 billion solar masses, assuming Eddington-limited accretion. This discovery adds significant weight to existing challenges in astrophysics about the rapid formation and growth of supermassive black holes in the early universe. This QSO's properties stand to inform models of cosmic reionization, metal enrichment studies, and measurements of cosmic expansion.

Future Research Directions

The study suggests further exploration in the field of high-redshift QSOs particularly using spectroscopic surveys that complement infrared and optical data to enhance the detection of these rare objects. With forthcoming surveys, enhanced by the capabilities of Gaia and other large-scale efforts like the Taipan survey, the researchers anticipate a more comprehensive catalog of QSOs that push the boundaries of luminosity and redshift, facilitating deeper insights into early universe cosmology and black hole evolution.

The paper by Wolf et al. provides crucial insights and methodologies for identifying and understanding highly luminous QSOs in the early universe. It heralds further discoveries and theoretical challenges that continue to refine our understanding of astrophysical phenomena in the high-redshift universe.