The Northern Sky Optical Cluster Survey I: Detection of Galaxy Clusters in DPOSS

Abstract

The Northern Sky Optical Cluster Survey is a project to create an objective catalog of galaxy clusters over the entire high-galactic-latitude Northern sky, with well understood selection criteria. We use the object catalogs generated from the Digitized Second Palomar Sky Survey (DPOSS, Djorgovski et al. 1999) as the basis for this survey. We apply a color criterion to select against field galaxies, and use a simple adaptive kernel technique to create galaxy density maps, combined with the bootstrap technique to make significance maps, from which density peaks are selected. This survey attempts to eliminate some of the subjective criteria and assumptions of past surveys, including detection by eye (Abell 1958, ACO 1989) and assumed luminosity functions and cluster profiles (PDCS, Postman et al. 1995). We also utilize more information (especially colors) than the most similar recent survey, the APM (Dalton et al. 1992). This paper presents the details of our cluster detection technique, as well as some initial results for two small areas totaling ~60 square degrees. We find a mean surface density of ~1.5 clusters per square degree, consistent with the detection of richness class 0 and higher clusters to z~0.3. In addition, we demonstrate an effective photometric redshift estimator for our clusters.