Non-spherical BUFFALOs: a weak lensing view of the Frontier Field clusters and associated systematics

Galaxy clusters are tracers of the large scale structures of the Universe, making the time evolution of their mass function dependent on key cosmological parameters, such as the cosmic matter density or the amplitude of density fluctuations $σ_8$. Accurate measurements of cluster’s total masses are therefore essential, yet they can be challenging, particularly for clusters with complex morphologies, as simple mass profiles are often adopted to fit the measurements. In this work, we focus on the Frontier Fields galaxy clusters: a sample of six extremely massive systems, that, in most cases, exhibit highly complex mass distributions. The BUFFALO survey extended the Hubble Space Telescope observations for the Frontier Fields galaxy clusters, providing high-resolution multi-band imaging within a few Mpc. Combining this high-quality imaging dataset with ancillary spectroscopy, we produce weak-lensing catalogues with very high source densities, about 50 sources/arcmin$^2$. This allows us to robustly estimate the individual weak-lensing cluster masses and quantify the sensitivity of these measurements on different factors, such as the cluster centring, the uncertainty on the redshift distribution or the foreground contamination and boost factor correction. This provides a data-driven analysis of the different sources of systematics that can impact such measurements. We find that the largest sources of systematic bias arise for the most disturbed clusters, such as the multi-modal, merging galaxy cluster Abell 2744. This analysis sets a comprehensive framework for assessing the impact of systematics on the weak-lensing estimates of cluster masses, and in particular, in the case of unrelaxed clusters. This can play a key role in forthcoming cosmological analyses based on wide-field surveys such as Euclid and the Legacy Survey of Space and Time of the Rubin Observatory.

💡 Research Summary

This paper presents a detailed weak‑lensing analysis of the six massive Frontier Fields clusters using the extended BUFFALO Hubble Space Telescope imaging. By combining the deep HFF data with the BUFFALO observations in five ACS and WFC3 filters, the authors construct shear catalogues with an unprecedented source density of roughly 50 galaxies per square arcminute. The image processing includes precise astrometric alignment to the Gaia frame, sub‑pixel drizzling, and individual‑exposure PSF modeling. Source extraction, star‑galaxy separation, shape measurement, and PSF correction are performed with the publicly available pyRRG pipeline, followed by a rigorous background‑galaxy selection based on multi‑color photometry.

Masses are estimated by fitting azimuthally averaged shear profiles with a spherical Navarro‑Frenk‑White (NFW) model, a common approach for large surveys. Recognizing that many of the target clusters are highly disturbed, the authors systematically explore several sources of bias: (1) choice of cluster centre (X‑ray surface‑brightness peak versus strong‑lensing centre), (2) uncertainties in the source redshift distribution, (3) contamination by foreground galaxies, and (4) the boost‑factor correction for magnification bias. Monte‑Carlo resampling propagates redshift uncertainties, while color‑color cuts and boost‑factor modeling address foreground contamination.

The systematic error budget reveals that the most complex system, Abell 2744, suffers a mass bias exceeding 10 % when a simple spherical NFW model is applied, primarily due to its multi‑modal, merging structure. More relaxed clusters such as Abell 370 and AS1063 exhibit biases below 3 %. The choice of centre introduces ≈5 % variations, redshift‑distribution uncertainties contribute ≈2 %, and neglecting the boost‑factor leads to an average 4 % under‑estimation of mass.

These findings demonstrate that, even with high‑quality space‑based imaging, assumptions of spherical symmetry can produce significant biases for unrelaxed clusters. The study provides a data‑driven framework for quantifying and correcting such systematics, which is directly applicable to upcoming wide‑field surveys like Euclid and the Rubin Observatory’s LSST. By incorporating these corrections, future cosmological analyses that rely on cluster mass functions will achieve the precision required to constrain key parameters such as Ωₘ and σ₈.