Quasars as standard candles

III. Validation of a new sample for cosmological studies^⋆

¹ Dipartimento di Fisica e Astronomia, Università di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Firenze, Italy
e-mail: elisabeta.lusso@unifi.it
² INAF – Osservatorio Astrofisico di Arcetri, 50125 Florence, Italy
³ Dipartimento di Fisica, Università degli studi di Napoli Federico II, Via Cinthia, 80126 Napoli, Italy
⁴ INFN – Sezione di Napoli, Via Cinthia 9, 80126 Napoli, Italy
⁵ Scuola Superiore Meridionale, Università di Napoli Federico II, Largo San Marcellino 10, 80138 Napoli, Italy
⁶ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica Milano, Via Corti 12, 20133 Milano, Italy
⁷ Gran Sasso Science Institute, Via F. Crispi 7, 67100 L’Aquila, Italy
⁸ Laboratory for Theoretical Cosmology, Tomsk State University of Control Systems and Radioelectronics (TUSUR), 634050 Tomsk, Russia
⁹ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
¹⁰ Yale University, Department of Computer Science, 51 Prospect St, New Haven, CT 06511, USA
¹¹ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, via Gobetti 93/3, 40129 Bologna, Italy
¹² Dipartimento di Astronomia, Università degli Studi di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
¹³ INAF – Osservatorio Astronomico di Capodimonte, Via Moiariello 16, 80131 Napoli, Italy

Received: 12 July 2020
Accepted: 19 August 2020

Abstract

We present a new catalogue of ∼2400 optically selected quasars with spectroscopic redshifts and X-ray observations from either Chandra or XMM–Newton. The sample can be used to investigate the non-linear relation between the ultraviolet (UV) and X-ray luminosity of quasars as well as to build a Hubble diagram up to a redshift of z ∼ 7.5. We selected sources that are neither reddened by dust in the optical and UV nor obscured by gas in the X-rays, and whose X-ray fluxes are free from flux-limit-related biases. After checking for any possible systematics, we confirm, in agreement with our previous works, that the X-ray to UV relation provides distance estimates matching those from supernovae up to z ∼ 1.5, and its slope shows no redshift evolution up to z ∼ 5. We provide a full description of the methodology for testing cosmological models, further supporting a trend whereby the Hubble diagram of quasars is well reproduced by the standard flat cold dark matter model up to z ∼ 1.5–2, but strong deviations emerge at higher redshifts. Since we have minimised all non-negligible systematic effects and proven the stability of the L_X − L_UV relation at high redshifts, we conclude that an evolution of the expansion rate of the Universe should be considered as a possible explanation for the observed deviation, rather than some systematic (redshift-dependent) effect associated with high-redshift quasars.