A new candidate quasar strongly lensed by the galaxy cluster WHJ0400-27 with an 18″ image separation

¹ Department of Physics and Earth Science, University of Ferrara, via Saragat 1, I-44122 Ferrara, Italy
² INAF – OAS, Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, via Gobetti 93/3, I-40129 Bologna, Italy
³ INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, I-80131 Napoli, Italy
⁴ Dipartimento di Fisica, Università degli Studi di Milano, via Celoria 16, I-20133 Milano, Italy
⁵ INAF – IASF Milano, via A. Corti 12, I-20133 Milano, Italy
⁶ Departamento de Física Moderna, Instituto de Fisica de Cantabria, Avd. Los Castros 48, 39005 Santander, Spain
⁷ Università di Firenze, Dipartimento di Fisica e Astronomia, via G. Sansone 1, 50019 Sesto F.no, Firenze, Italy
⁸ INAF – Osservatorio Astrofisico di Arcetri, Via Largo E. Fermi 5, 50125 Firenze, Italy
⁹ Zentrum für Astronomie, Universität Heidelberg, Philosophenweg 12, D-69120 Heidelberg, Germany
¹⁰ ITP, Universität Heidelberg, Philosophenweg 16, D-69120 Heidelberg, Germany
¹¹ INAF – Osservatorio Astronomico di Padova, vicolo dell’Osservatorio 5, I-35122 Padova, Italy
¹² Università di Salerno, Dipartimento di Fisica “E.R. Caianiello”, Via Giovanni Paolo II 132, I-84084 Fisciano (SA), Italy
¹³ University of Trento, Via Sommarive 14, I-38123 Trento, Italy
¹⁴ INFN – Gruppo Collegato di Salerno – Sezione di Napoli, Dipartimento di Fisica “E.R. Caianiello”, Università di Salerno, via Giovanni Paolo II, 132, I-84084 Fisciano (SA), Italy

^⋆ Corresponding author; bzzlnz@unife.it

Received: 3 July 2024
Accepted: 9 April 2025

Abstract

Context. Time-delay cosmography (TDC) using quasi-stellar objects (QSOs) multiply lensed by galaxies has recently emerged as an independent and competitive tool for measuring the value of the Hubble constant. Lens galaxy clusters hosting multiply imaged QSOs, when coupled with an accurate and precise knowledge of their total mass distribution, are equally powerful cosmological probes. However, fewer than ten such systems have been identified to date.

Aims. Our study aims to expand the limited sample of cluster-lensed QSO systems by identifying new candidates within rich galaxy clusters.

Methods. We started with a sample of approximately 10⁵ galaxy cluster candidates from Dark Energy Survey and Wide-field Infrared Survey Explorer imaging data, along with a pure catalogue of over one million QSOs from Gaia DR3. We cross-correlated these datasets to identify lensed QSO candidates near the cores of massive galaxy clusters.

Results. Our search detected three lensed double candidates across an area of ≈5000 sq degree. In this work, we focus on the best candidate – a double QSO with a Gaia-based redshift of 1.35, projected behind the moderately rich cluster WHJ0400-27 at z_phot = 0.65. Based on a first spectroscopic follow-up study, we confirm the two QSOs at z = 1.345 with indistinguishable spectra, and a brightest cluster galaxy at z = 0.626. These observations support the strong lensing nature of this system, though some tension arises when the cluster mass from the preliminary lens model is compared to other mass proxies. We also considered whether such a system could be a rare physical association of two distinct QSOs separated by a projected physical distance of ≈150 kpc. If further spectroscopic observations confirm its lensing nature, such a rare lens system would exhibit one of the largest image separations observed to date (Δϑ = 17.8″) and would enable interesting TDC applications.