Cosmology-dependent covariance in galaxy cluster number counts: consequences for parameter inference
Abstract
Galaxy clusters provide constraints on cosmology through their abundance as a function of mass and redshift. Parameter inference from cluster counts requires modelling the covariance entering the likelihood, including contributions from Poisson shot noise and super-sample covariance (SSC) induced by long-wavelength density fluctuations. Since evaluating the full covariance during parameter inference can be computationally expensive, many analyses compute it at a fiducial cosmology and keep it fixed. In this work, we investigate the impact of covariance misspecification on the estimation of Ωc, σ8, and w. We perform a systematic analysis in which the covariance is either varied consistently with the sampled cosmology or fixed at displaced cosmological models, including intermediate strategies where only selected components, such as SSC, are held fixed. Our analysis incorporates observational effects relevant for LSST-like optical surveys, including mass-proxy scatter and photometric redshift uncertainties. We find that the estimators of Ωc, σ8, and w remain unbiased even when the covariance is evaluated at an incorrect cosmology. However, fixing the covariance can significantly over- or underestimate confidence regions. The magnitude and sign of this effect are primarily driven by parameters controlling the amplitude of matter fluctuations. For LSST-like surveys, an inconsistent covariance specification can artificially modify the apparent S8 tension inferred from cluster counts. We further show that a single covariance update evaluated at the recovered best-fit cosmology is sufficient to restore the correct uncertainty normalization. These results indicate that fixed-covariance approximations may be adequate for some single-probe analyses, but a fully cosmology-dependent treatment is required for consistent multi-probe studies. ABRIDGED
Versions
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BibTeX
@misc{lettieri2026cosmologydependent,
title = {{Cosmology-dependent covariance in galaxy cluster number counts: consequences for parameter inference}},
author = {Henrique C. N. Lettieri and Mariana Penna-Lima and Sandro D. P. Vitenti},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2606.30459},
eprint = {2606.30459},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2606.30459},
}
RIS
TY - UNPB
TI - Cosmology-dependent covariance in galaxy cluster number counts: consequences for parameter inference
AU - Henrique C. N. Lettieri
AU - Mariana Penna-Lima
AU - Sandro D. P. Vitenti
PY - 2026
DA - 2026/09/21
PB - arXiv
DO - 10.48550/arxiv.2606.30459
UR - https://arxiv.org/abs/2606.30459
AB - Galaxy clusters provide constraints on cosmology through their abundance as a function of mass and redshift. Parameter inference from cluster counts requires modelling the covariance entering the likelihood, including contributions from Poisson shot noise and super-sample covariance (SSC) induced by long-wavelength density fluctuations. Since evaluating the full covariance during parameter inference can be computationally expensive, many analyses compute it at a fiducial cosmology and keep it fixed. In this work, we investigate the impact of covariance misspecification on the estimation of Ω_c, σ_8, and w. We perform a systematic analysis in which the covariance is either varied consistently with the sampled cosmology or fixed at displaced cosmological models, including intermediate strategies where only selected components, such as SSC, are held fixed. Our analysis incorporates observational effects relevant for LSST-like optical surveys, including mass-proxy scatter and photometric redshift uncertainties. We find that the estimators of Ω_c, σ_8, and w remain unbiased even when the covariance is evaluated at an incorrect cosmology. However, fixing the covariance can significantly over- or underestimate confidence regions. The magnitude and sign of this effect are primarily driven by parameters controlling the amplitude of matter fluctuations. For LSST-like surveys, an inconsistent covariance specification can artificially modify the apparent S_8 tension inferred from cluster counts. We further show that a single covariance update evaluated at the recovered best-fit cosmology is sufficient to restore the correct uncertainty normalization. These results indicate that fixed-covariance approximations may be adequate for some single-probe analyses, but a fully cosmology-dependent treatment is required for consistent multi-probe studies. ABRIDGED
ER -