SPECTROSCOPIC RICHNESS OF CARDINAL GALAXY CLUSTERS

Abstract

This paper describes a method to compute the spectroscopic richness of galaxy clusters. This method stems from the redMaPPer photometric cluster finder and the idea that spectroscopic data is able to better discriminate between member and non-member galaxies. Similar to redMaPPer, the method outlined here defines a membership probability for each galaxy around a central galaxy of a cluster. However instead of utilizing the position, color, and magnitude information of the galaxy, the spectroscopic method utilizes the velocity information of galaxies around a cluster to determine if galaxies with a given position and velocity are likely to be cluster members or not. Running on halos from the Cardinal mock galaxy catalog and determining the richness of the cluster in each halo yields a mass-richness relationship. Comparing the mass-richness relation found from this spectroscopic method and the relation from redMaPPer yields a clear decrease in the scatter of 15 percent at low mass and 25 percent at high mass. This reveals the promise of incorporating a spectroscopic component into cluster finder algorithms.