Spectroscopic Characterization of redMaPPer Galaxy Clusters with DESI
Abstract
Optical galaxy cluster identification algorithms such as redMaPPer promise to enable an array of astrophysical and cosmological studies, but suffer from biases whereby galaxies in front of and behind a galaxy cluster are mistakenly associated with the primary cluster halo. These projection effects caused by irreducible photometric redshift uncertainty must be quantified to facilitate the use of optical cluster catalogues. We present measurements of galaxy cluster projection effects and velocity dispersion using spectroscopy from the Dark Energy Spectroscopic Instrument (DESI). Our findings are as follows: we confirm that the fraction of redMaPPer putative member galaxies mistakenly associated with cluster haloes is richness dependent, being more than twice as large at low richness than high richness; we present the first spectroscopic evidence of an increase in projection effects with increasing redshift, by as much as 25 per cent from to ; moreover, we find qualitative evidence for luminosity dependence in projection effects, with fainter galaxies being more commonly far behind clusters than their bright counterparts; finally we fit the scaling relation between measured mean spectroscopic richness and velocity dispersion, finding an implied linear scaling between spectroscopic richness and halo mass. We discuss further directions for the application of spectroscopic datasets to improve use of optically selected clusters to test cosmological models.
Cite
@article{arxiv.2506.06249,
title = {Spectroscopic Characterization of redMaPPer Galaxy Clusters with DESI},
author = {J. Myles and D. Gruen and T. Jeltema and A. Mantz and S. Allen and S. Fu and A. Kremin and J. Aguilar and S. Ahlen and D. Bianchi and D. Brooks and F. J. Castander and T. Claybaugh and A. de la Macorra and A. Dey and P. Doel and S. Ferraro and J. E. Forero-Romero and E. Gaztañaga and S. Gontcho A Gontcho and G. Gutierrez and K. Honscheid and M. Ishak and R. Kehoe and D. Kirkby and T. Kisner and O. Lahav and M. Landriau and L. LeGuillou and M. Manera and A. Meisner and R. Miquel and J. Moustakas and S. Nadathur and J. A. Newman and N. Palanque-Delabrouille and F. Prada and I. Pérez-Ràfols and G. Rossi and E. Sanchez and D. Schlegel and M. Schubnell and J. Silber and D. Sprayberry and G. Tarlé and B. A. Weaver and R. Zhou},
journal= {arXiv preprint arXiv:2506.06249},
year = {2025}
}