Probing Projection Effects in Optically-Selected Clusters with Velocity Dispersions Outliers
Abstract
The evolution of galaxy cluster abundance is a powerful probe of cosmology, but its efficacy depends on our ability to understand and control for systematics in the selection and mass estimation of clusters. We study redMaPPer-selected clusters in the Cardinal simulation, which simulates the DES Y3 observational conditions. Specifically, we investigate the line-of-sight velocity distributions of cluster member galaxies in 12,272 simulated clusters. We find that a significant fraction of clusters, 35%, have velocity dispersions that are high compared to their richness with an offset of ∼ 400 km s−1 between the outliers and the main population. A similar population of velocity dispersion outliers was found in the DES Y3 data. Nearly all of the outliers (98%) in a low-richness subsample have non-Gaussian velocity distributions, and they are more likely than the main population to have significant velocity substructure. These clusters also tend to have higher redshifts than clusters with typical velocity dispersions, and they show a larger scatter between halo mass and richness. Overall, our results point to a significant contamination of redMaPPer-selected samples from line of sight structure, particularly at high redshifts and low richness. Comparison to the Buzzard simulations shows similar results.
Related
- Optical Depths from the Thermal Sunyaev-Zel'dovich Effect with ACT DR6 and DESI DR1 Spectroscopic Galaxies and Optically-Selected Clusters
- Forecasting the Resolvability of Stellar Velocity Dispersions in Ultra-faint Dwarf Galaxies
- Optically Selected Superthin Galaxies Remain Thin in the Near-infrared
- CHEX-MATE: AMALGAM weak-lensing analysis of 41 Planck Sunyaev--Zel'dovich-selected galaxy clusters
- A DECADE of Giants: A catalog of 42,000 optically identified galaxy clusters from 13,000 deg2 of the sky
BibTeX
@misc{graham2026probing,
title = {{Probing Projection Effects in Optically-Selected Clusters with Velocity Dispersions Outliers}},
author = {K Graham and T.E Jeltema and C.-H To and V Wetzell and Kyle W Davis and Conghao Zhou and H.-Y Wu and E.S Rykoff and J Derose and Michel Aguena and R.H Wechsler},
howpublished = {HAL},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.08095},
eprint = {2609.08095},
archivePrefix = {arXiv},
url = {https://hal.science/hal-05759376v1},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Probing Projection Effects in Optically-Selected Clusters with Velocity Dispersions Outliers
AU - K Graham
AU - T.E Jeltema
AU - C.-H To
AU - V Wetzell
AU - Kyle W Davis
AU - Conghao Zhou
AU - H.-Y Wu
AU - E.S Rykoff
AU - J Derose
AU - Michel Aguena
AU - R.H Wechsler
PY - 2026
DA - 2026/09/08
PB - HAL
DO - 10.48550/arxiv.2609.08095
UR - https://hal.science/hal-05759376v1
AB - The evolution of galaxy cluster abundance is a powerful probe of cosmology, but its efficacy depends on our ability to understand and control for systematics in the selection and mass estimation of clusters. We study redMaPPer-selected clusters in the Cardinal simulation, which simulates the DES Y3 observational conditions. Specifically, we investigate the line-of-sight velocity distributions of cluster member galaxies in 12,272 simulated clusters. We find that a significant fraction of clusters, 35%, have velocity dispersions that are high compared to their richness with an offset of ∼ 400 km s^−1 between the outliers and the main population. A similar population of velocity dispersion outliers was found in the DES Y3 data. Nearly all of the outliers (98%) in a low-richness subsample have non-Gaussian velocity distributions, and they are more likely than the main population to have significant velocity substructure. These clusters also tend to have higher redshifts than clusters with typical velocity dispersions, and they show a larger scatter between halo mass and richness. Overall, our results point to a significant contamination of redMaPPer-selected samples from line of sight structure, particularly at high redshifts and low richness. Comparison to the Buzzard simulations shows similar results.
ER -