1. Rachel Anderson Laura Parker William Harris Department of Physics & Astronomy, McMaster University Hamilton, Ontario, L8S-4M1, Canada Searching for Galaxy Groups in Photometric Data from the CFHT Legacy Survey Objectives and Motivation In this study we search for over-densities of galaxies in the 4 Canada- France-Hawaii Telescope Legacy Survey (CFHTLS) Deep Fields using two methods:  A 5th Nearest Neighbor algorithm  A variation of the Probability Friends of Friends algorithm suggested by Li & Yee (2008) The results will be used to study galaxy properties as a function of environment. The CFHTLS is a joint effort between Canada and France to use ~50% of their dark and grey time on the Canada-France- Hawaii Telescope for a 5 year survey which began in 2003. The 4 CFHTLS Deep Fields Probability Friends of Friends 5th Nearest Neighbor Plots of 5th Nearest Neighbor Results Conclusion Acknowledgements The second method used is a modification of Friends of Friends: an iterative method of finding galaxy groups with spectroscopic redshifts (Huchra & Geller 1982). We use a variation of the Probability Friends of Friends algorithm suggested by Li & Yee (2008). What is unique about this method is that it can be used with photometric redshift data! Our approach employs a continuous friends-of-friends search in the transverse direction and uses the probability density in redshift space. The probability density utilizes the following definitions: P group (z) is the probability that galaxies A, B, …,n, are all at the same redshift, z, and P A (z) is the probability density of galaxy A defined by the uncertainty in the galaxy's redshift (  A ) and the redshift of A. The group redshift is that which maximizes P group (z). Also, where maxP is the what the numerator would be if all the galaxies were at the same redshift. For a galaxy to be included in a group, it must satisfy two conditions:  The separation between that galaxy and any member of the group must be less than 0.25 Mpc in the linear direction.  P ratio  P ratio,crit, where P ratio,crit is met when | z 1 - z 2 | =  1 +  2. If a galaxy meets this criteria for two or more groups, it belongs to the group with which it has the largest P ratio. Our Data Selection Criteria:  Redshift of 0.2  z  1.0  At least three bands were used in the computation of the redshift.  Luminosity cut of: i’ < 3.2080  10 50 Based on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/DAPNIA, at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council (NRC) of Canada, the Institut National des Science de l'Univers of the Centre National de la Recherche Scientifique (CNRS) of France, and the University of Hawaii. This work is based in part on data products produced at TERAPIX and the Canadian Astronomy Data Centre as part of the Canada-France-Hawaii Telescope Legacy Survey, a collaborative project of NRC and CNRS. References: CFHTLS, 2008 Cooper, M. C., Newman, J. A., Madgwick, D. S., Gerke, B. F., Yan, R., & Davis, M. 2005, ApJ, 634, 833 Huchra, J. P., & Geller, M. J. 1982, ApJ, 257, 423 Li, I. H., & Yee, H. K. C. 2008, AJ, 135, 809 5th Nearest Neighbor is one of the best ways (if not the best) to find over-densities (Cooper et al. 2005). We search for the 5th nearest neighbor in the transverse direction. In the radial direction, we only considered galaxies (galaxy 1 and galaxy 2) to be close neighbors if: | z 1 - z 2 | 2   1 2 +  2 2 The simple algorithm is as follows:  For each galaxy in a field, search through all the other galaxies in that field  Check that | z 1 - z 2 | 2   1 2 +  2 2, if not go to next galaxy  Calculate the angular separation between the two galaxies  Determine the distance to the 5th nearest neighbor -17:43:5622:15:31D4 +52:40:5614:19:27D3 +02:12:3010:00:28D2 -04:29:4002:25:59D1 DECRAField Table 1: CFHTLS Deep Fields. Each one is 1  1 . Image: CFHTLS Deep Field 1, courtesy of Stephen Gwyn Figure 3: Location of galaxies in field D1 with distances to 5th nearest neighbor increasing from 0.00085 degrees to 0.01748 degrees moving clockwise from upper left. The holes are masked regions. Galaxies with closer 5th Nearest Neighbors show clustering, whereas galaxies with larger separations show a more uniform distribution. Exactly what you would expect! 1 Mpc Figure 4: Redshift slices are taken for galaxies with nearest 5th neighbor (shown in upper left graph of Figure 2). Circles with radii of 1 Mpc are drawn for the average redshift of the different slices. We can see that high density regions extend across redshift slices. The holes are masked regions. We have presented two methods to determine galaxy densities. The Probability Friends-of-Friends algorithm is a useful group identifying tool, unique in that it allows for the use of photometric redshifts. However, it presents little numerical analysis on the density of the environment of an individual galaxy, as even some members of large groups must dwell on the group perimeter. The 5th Nearest Neighbor method yields just the opposite, allowing us to classify each individual galaxy based on its own environment, but make no group definitions. Figure 1: CFHT, courtesy of Charlie Kaminski Rachel Anderson anderre@mcmaster.ca Data is taken with the wide field imager MegaPrime, with a 36 CCD 1  1  field of view camera, MegaCam. The filters used for the 4 Deep fields are u*, g’, r’, i’, and z’, with a limiting magnitude of i’ ~ 25 (CFHTLS 2008). The CFHTLS Deep Fields are located at: Figure 2: Histogram of the separation between each galaxy and its 5th nearest neighbor.