The Impact of Cosmic Variance and Satellites on JWST Clustering Measurements at Redshift around 6

Jiamu Huang, Elia Pizzati, Joseph F. Hennawi, Joop Schaye, Matthieu Schaller, Benjamin Snyder, Yi Kang

First listed 2026-05-11 | Last updated 2026-05-11

Abstract

We present a framework for inferring the dark matter halo masses of quasars and [O III]-emitting galaxies from JWST/NIRCam Wide Field Slitless Spectroscopy (WFSS) clustering measurements at z approximately 6. Using the FLAMINGO-10k N-body simulation, we construct mock realizations of quasar and galaxy catalogs that incorporate realistic selection functions, spatial coverage, and sensitivity limits matched to the ASPIRE survey. These mocks enable accurate measurements of the quasar-galaxy cross-correlation and galaxy auto-correlation functions, with covariance matrices derived from 1000 realizations that capture both cosmic variance and bin-to-bin correlations. We employ Bayesian inference to fit the correlation functions and infer the minimum halo masses for quasars and galaxies. Our results demonstrate that Poisson pair-count uncertainties, commonly adopted in high-redshift clustering studies, significantly underestimate the true measurement errors. The dominant missing component is cosmic variance: even the diagonal of the full covariance matrix exceeds the Poisson expectation, with off-diagonal bin-to-bin correlations contributing a smaller additional correction. In particular, 1) the commonly used Poisson error on the correlation functions underestimates the true uncertainty by a factor of approximately 3; 2) the uncertainties on the inferred minimum halo masses are underestimated by a factor of approximately 1.5-3 when adopting Poisson errors instead of the full covariance matrix; 3) the inferred QSO halo mass is robust to whether central and satellite [O III]-emitters share a common mass threshold. Our framework provides a more complete error budget for JWST/WFSS clustering analyses, enabling robust constraints on the host halo masses and duty cycles of high-redshift quasars and emission-line galaxies.

Short digest

This paper builds an ASPIRE-matched clustering inference framework for z≈6 quasars and [O III]-emitting galaxies using FLAMINGO-10k mock catalogs with realistic JWST/NIRCam WFSS selection, coverage, and sensitivity. From 1000 realizations, the authors estimate full covariance matrices for the quasar-galaxy cross-correlation and galaxy auto-correlation, then use Bayesian fits to recover minimum host halo masses. The main result is that the usual Poisson pair-count errors miss the dominant contribution from cosmic variance, underestimating correlation-function uncertainties by about a factor of 3 and halo-mass uncertainties by roughly 1.5 to 3. That matters directly for interpreting early quasar environments, because the inferred QSO halo mass remains robust even if central and satellite [O III] emitters are assigned different mass-threshold assumptions, giving a firmer path to halo-mass and duty-cycle constraints from JWST clustering data.

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