Volume 642, October 2020
|Number of page(s)||24|
|Published online||13 October 2020|
Quasars as standard candles
III. Validation of a new sample for cosmological studies⋆
Dipartimento di Fisica e Astronomia, Università di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Firenze, Italy
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
Accepted: 19 August 2020
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 LX − LUV 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.
Key words: galaxies: active / quasars: general / quasars: supermassive black holes / methods: statistical
Full Table 3 is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (220.127.116.11) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/642/A150
© ESO 2020
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