Quasar standardization: Overcoming Selection Biases and Redshift Evolution

TL;DR

This paper demonstrates that the UV-X-ray flux relation in quasars can be corrected for biases and redshift evolution, making QSOs reliable standard candles for cosmology up to very high redshifts.

Contribution

It introduces a statistical correction method that confirms the intrinsic nature of the UV-X-ray flux relation in QSOs, supporting their use as standard candles.

Findings

The UV-X-ray flux relation is robust after bias correction.

QSOs can be standardized as cosmological probes up to redshift 7.54.

The corrected relation matches previous parameter estimates.

Abstract

Quasars (QSOs) are extremely luminous active galatic nuclei currently observed up to redshift $z=7.642$. As such, they have the potential to be the next rung of the cosmic distance ladder beyond SNe Ia, if they can reliably be used as cosmological probes. The main issue in adopting QSOs as standard candles (similarly to Gamma-Ray Bursts) is the large intrinsic scatter in the relations between their observed properties. This could be overcome by finding correlations among their observables that are intrinsic to the physics of QSOs and not artifacts of selection biases and/or redshift evolution. The reliability of these correlations should be verified through well-established statistical tests. The correlation between the ultraviolet (UV) and X-ray fluxes developed by Risaliti \& Lusso is one of the most promising relations. We apply a statistical method to correct this relation for redshift evolution and selection biases. \textbf{Remarkably, we recover the the same parameters of the slope and the normalization as Risaliti \& Lusso. Our results establish the reliability of this relation, which is intrinsic to the QSO properties and not merely an effect of selection biases or redshift evolution. Hence,} the possibility to standardize QSOs as cosmological candles, thereby extending the Hubble diagram up to $z=7.54$.