We present a structure-finding algorithm designed to identify galaxy groups in photometric-redshift data sets: the probability friends-of-friends (pFoF) algorithm. This algorithm is derived by combining the FoF algorithm in the transverse direction and the photometric-redshift probability densities in the radial dimension. The innovative characteristic of our group-finding algorithm is the improvement of redshift estimation via the constraints given by the transversely connected galaxies in a group, based on the assumption that all galaxies in a group have the same redshift. Tests using the Virgo Consortium Millennium Simulation mock catalogs allow us to show that the recovery rate of the pFoF algorithm is larger than 80% for mock groups of at least 2 × 1013 M☉, while the false detection rate is about 10% for pFoF groups containing around at least eight net members. Applying the algorithm to the Canadian Network for Observational Cosmology Survey group catalogs gives results which are consistent with the mock catalog tests. From all these results, we conclude that our group-finding algorithm offers an effective yet simple way to identify galaxy groups in photometric-redshift catalogs.
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