Quasar pairs as large-scale structure tracers

¹ Departamento de Física, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria 1428, Pabellón I, Buenos Aires, Argentina
² Núcleo Cosmo-ufes & Departamento de Física – Universidade Federal do Espírito Santo, 29075-910 Vitória, ES, Brazil
³ Instituto de Astronomía Teórica y Experimental (IATE), CONICET-UNC, Córdoba, Argentina
⁴ Observatorio Astronómico de Córdoba, UNC, Córdoba, Argentina
⁵ Comisión Nacional de Actividades Espaciales (CONAE), Córdoba, Argentina
⁶ Departamento de Física, Universidade Estadual de Londrina, Rod. Celso Garcia Cid, Km 380, 86057-970 Londrina, PR, Brazil

^⋆ Corresponding author: martin@df.uba.ar

Received: 2 April 2025
Accepted: 9 July 2025

Abstract

Aims. Quasars can be used as suitable tracers of the large-scale distribution of galaxies at high redshift given their high luminosity and dedicated surveys. Previous studies have shown that quasars have a bias similar to that of rich groups. Following this argument, quasar pairs could be associated with higher-density environments serving as protocluster proxies.

Methods. In this work, our aim is to characterize close quasar pairs residing in the same halo. This is accomplished by identifying quasar pairs in redshift space. We analyze pair-quasar cross correlations, as well as cosmic microwave background (CMB)-derived lensing convergence profiles centered on quasar and quasar pairs.

Results. We identified quasar pairs in the SDSS-DR16 catalog as objects with relative velocities within |ΔV|≤900 km/s and projected separation less than d_p ≤ 2 Mpc, in the redshift range 1.2 ≤ z ≤ 2.8. We computed redshift-space cross-correlation functions using Landy-Szalay estimators for the samples. For the analysis of the correlation between quasars or quasar pairs and the underlying mass distribution, we calculated mean radial profiles of the lensing convergence parameter using CMB data provided by the Planck Collaboration.

Conclusions. We identified 2777 pairs of quasars in the redshift range 1.2–2.8. Quasar pairs show a distribution of relative luminosities that differs from that found in two pairs selected at random with the same redshift distribution, indicating that quasars in these systems distinguish themselves from isolated ones. The cross-correlation between pairs and quasars shows a larger correlation amplitude than the autocorrelation function of quasars, indicating that these systems are more strongly biased with respect to the large-scale mass distribution and reside in more massive halos. This is reinforced by the higher convergence CMB lensing profiles of the pairs compared to isolated quasars with a similar redshift distribution. Our results show that quasar pairs are suitable precursors of present-day clusters of galaxies, in contrast to isolated quasars, which are associated with more moderate density environments.