Finding structures in photometric redshift galaxy surveys: an extended friends-of-friends algorithm

Abstract

We present a modified version of the friends-of-friends (FOF) structure-finding algorithm, designed specifically to locate groups or clusters of galaxies in photometric redshift data sets. The main objective of this paper is to show that this extended friends-of-friends (hereafter EXT-FOF) algorithm yields results almost identical to the original FOF, if applied to a spectroscopic redshift data set, and a rather conservative catalogue of structures, in case of a data set with simulated photometric redshifts. Therefore, we create group catalogues for the first Center for Astrophysics Redshift Survey (CFA1), as well as for the Las Campanas Redshift Survey (LCRS), both of which being spectroscopic surveys, using FOF algorithms. We then apply our new algorithm to said surveys and compare the resulting structure catalogue. Furthermore, we bestow simulated photometric redshifts on the LCRS galaxies, and use the EXT-FOF to detect structures, which we compare in size and composition to the ones found in the original, spectroscopic data set. We will show that the properties of this modified algorithm are well understood and that it is suited for finding structures in photometric data sets. This is the first paper in a series of papers, dealing with the application of our new cluster finding algorithm to various photometric redshift galaxy surveys.