Calibrating optical galaxy cluster projection effects with sparse spectroscopic samples: A clustering redshift approach
Phys. Rev. D 114, 083530 – Published 19 October, 2026
DOI: https://doi.org/10.1103/j128-fvwd
Abstract
Wide-field optical imaging surveys are efficient at identifying galaxy clusters, but optically identified clusters suffer from projection effects—physically unassociated galaxies along the line of sight are often included as cluster members due to distance uncertainties. Previous studies have used spectroscopic follow-up observations of cluster members to quantify projection effects; however, such follow-up efforts cannot keep pace with the rapidly growing cluster samples. On the other hand, spectroscopic surveys designed for large-scale structure studies collect tens of millions of spectra but tend to have sparse spectra in cluster regions. To bridge this gap, we develop a clustering redshift approach that cross-correlates cluster members with sparse, noncluster-targeted spectroscopic galaxy samples. We validate this approach using the Cardinal simulation, recovering the correct spectroscopic redshift distribution and projection effect parameters of cluster members identified by the red-sequence Matched-filter Probabilistic Percolation algorithm. Our approach is insensitive to the selection of the spectroscopic sample and paves the way for calibrating projection effects in the upcoming Legacy Survey of Space and Time clusters using Dark Energy Spectroscopic Instrument and Roman spectroscopic samples.
