Spectroscopic Characterization of redMaPPer Galaxy Clusters with DESI

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. 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 $z\sim 0.1$ to $z\sim 0.2$; 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 data sets to improve use of optically selected clusters to test cosmological models.. Authors: Myles, J [Princeton University Department of Astrophysical Sciences, , Princeton NJ 08544 ,] (ORCID:0000000161455859); Gruen, D [Excellence Cluster ORIGINS , Boltzmannstrasse 2, D-85748 Garching ,; University Observatory, Faculty of Physics, Ludwig-Maximilians-Universität , Scheinerstr. 1, D-81677 München ,] (ORCID:0000000332707644); Jeltema, T [University of California Department of Astronomy and Astrophysics, UCO/Lick Observatory, , 1156 High Street, Santa Cruz, CA 95064 ,] (ORCID:0000000160890365); Mantz, A [Stanford University Physics Department, , Stanford, CA 93405 ,; SLAC National Accelerator Laboratory , 2575 Sand Hill Road, Menlo Park, CA 94025 ,] (ORCID:0000000280311217); Allen, S [Stanford University Physics Department, , Stanford, CA 93405 ,; SLAC National Accelerator Laboratory , 2575 Sand Hill Road, Menlo Park, CA 94025 ,]. DOE Contract: 89243024CSC000002; AC02-05CH11231; SC0009920; SC0019193; SC0022184. Subjects: clusters; galaxies; galaxies: clusters: general; galaxies: general; galaxies: groups: general; large-scale structure of Universe; large-scale structure of the universe