Group-finding with photometric redshifts: The Photo-z Probability Peaks
  algorithm
  We present a galaxy group-finding algorithm, the Photo-z Probability Peaks
(P3) algorithm, optimized for locating small galaxy groups using photometric
redshift data by searching for peaks in the signal-to-noise of the local
overdensity of galaxies in a three-dimensional grid. This method is an
improvement over similar two-dimensional matched-filter methods in reducing
background contamination through the use of redshift information, allowing it
to accurately detect groups at lower richness. We present the results of tests
of our algorithm on galaxy catalogues from the Millennium Simulation. Using a
minimum S/N of 3 for detected groups, a group aperture size of 0.25 Mpc/h, and
assuming photometric redshift accuracy of sigma_z = 0.05 it attains a purity of
84% and detects ~295 groups/deg.^2 with an average group richness of 8.6
members. Assuming photometric redshift accuracy of sigma_z = 0.02, it attains a
purity of 97% and detects ~143 groups/deg.^2 with an average group richness of
12.5 members. We also test our algorithm on data available for the COSMOS field
and the presently-available fields from the CFHTLS-Wide survey, presenting
preliminary results of this analysis.
