The Splashback Mass Function of Galaxy Clusters from Photometric Data

TL;DR

This paper introduces a photometric method to measure galaxy cluster splashback radii and masses, constructing an observational splashback mass function from SDSS data, which aligns with simulations at high masses.

Contribution

It develops a novel photometric framework for measuring splashback properties and mass functions, enabling cosmological studies without spectroscopic or lensing data.

Findings

Median splashback radius ratio R_sp/R_200m ≈ 1.1, consistent with previous studies.

Recalibrated the M_sp–R_sp scaling relation over 0.01 < z < 0.8, with a shallower slope.

Constructed the first observational splashback mass function from photometric data, matching simulations at high masses.

Abstract

The splashback radius marks the physical boundary of galaxy clusters, separating orbiting from infalling material, and provides a halo definition free from pseudo-evolution. In this work, we present a fully photometric framework to measure individual cluster splashback radii and masses, and to construct an observational splashback mass function. Using Sloan Digital Sky Survey data, we develop a probabilistic cluster membership method based on radial and photometric redshift information, optimized through an adaptive probability cut that maximizes the detection significance of the cluster core relative to its outskirts. We apply this methodology to a sample of 499 galaxy clusters from the \textsc{CoMaLit} weak-lensing compilation and recover splashback radii from modeling cumulative galaxy number profiles. The resulting splashback radii exhibit a median ratio $R_{\mathrm{sp}}/R_{200\mathrm{m}} \simeq 1.1$, consistent with previous observational studies. Using these measurements, we recalibrate the $M_{\mathrm{sp}}$--$R_{\mathrm{sp}}$ scaling relation over a wide redshift range ($0.01 < z < 0.8$), finding a slope shallower than the constant-density expectation and no significant redshift evolution. We then apply this relation to \textsc{redMaPPer} clusters in the SDSS Northern Galactic Cap to derive splashback masses for more than $1.5\times10^4$ systems and construct the first observational splashback mass function based solely on photometric data. The resulting mass function agrees with simulation-based predictions at the high-mass end, while deviations at lower masses are consistent with known completeness limits of optical cluster catalogs. Our results demonstrate that splashback-based cluster sizes, masses, and abundances can be robustly measured in photometric surveys, enabling cosmological studies without spectroscopic or lensing data.