A first look at quasar-galaxy clustering at z  ≃  7.3

¹ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, D-21029 Hamburg, Germany
² Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA, Leiden, The Netherlands
³ Department of Physics, Broida Hall, University of California, Santa Barbara, Santa Barbara, CA 93106-9530, USA
⁴ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁵ Institute for Theoretical Physics, Heidelberg University, Philosophenweg 12, D-69120 Heidelberg, Germany
⁶ Department of Astronomy, University of Michigan, 1085 S. University Ave., Ann Arbor, MI 48109, USA
⁷ Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
⁸ MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
⁹ Steward Observatory, University of Arizona, 933 N Cherry Ave, Tucson, AZ 85721, USA
¹⁰ Cosmic Dawn Center (DAWN), Niels Bohr Institute, University of Copenhagen, Jagtvej 128, DK-2200 København N, Denmark

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Received: 9 October 2025
Accepted: 17 February 2026

Abstract

Linking quasars to their dark matter environments provides critical insights into the formation and early growth of supermassive black holes (SMBHs). We present JWST observations of the environments surrounding two high-redshift quasars, J0252−0503 at z = 7.0 and J1007+2115 at z = 7.5, which enable the first constraints on quasar–galaxy clustering at z ≃ 7.3. Galaxies in the vicinity of the quasars were selected through ground-based and JWST/NIRCam imaging and were then spectroscopically confirmed with JWST/NIRSpec using the multi-shutter assembly (MSA). Over both fields, we identified 51 z > 5 galaxies, of which eight are found within a Δv_LOS = ± 1500 km s⁻¹ line-of-sight velocity window from the quasars and another eight in the background. The galaxy J0252_8713, located just 7 pkpc and Δv_LOS ≈ 360 km s⁻¹ from quasar J0252−0503, emerges as a compelling candidate for one of the most distant quasar-galaxy mergers. Combining the galaxy discoveries over the two fields, we measured the quasar-galaxy cross-correlation and obtain a correlation length of r₀^QG ≈ 7.6^+1.7_−1.6 h⁻¹ cMpc, based on a power-law model with a fixed slope of γ_QG = 2.0. Under the assumption that quasars and galaxies trace the same underlying dark matter density fluctuations, we infer a minimum dark matter halo mass for z ≃ 7.3 quasars of log₁₀(M_halo,min/M_⊙) = 11.6^+0.6_−0.7 in a halo model framework. Compared to measurements from EIGER at ⟨z⟩ = 6.25 and ASPIRE at ⟨z⟩ = 6.7 (where log₁₀(M_halo, min/M_⊙)≳12.1), our clustering results provide tentative evidence for a nonmonotonic redshift evolution of quasar clustering properties. We further estimate a quasar duty cycle of f_duty ≈ 0.05%, consistent with constraints from quasar proximity zones and intergalactic medium (IGM) damping wings. However, this implies very short phases of quasar activity, exacerbating the challenge to build billion solar mass SMBHs in only 700 Myr of cosmic time.