SEARCHING FOR GALAXY GROUPS IN PHOTOMETRIC DATA FROM THE CFHT LEGACY SURVEY

Abstract

<p>Most galaxies in the universe live in galaxy groups, and we can use them to learn a lot about t he influence of environment on galaxy evolution. Historically, galaxy groups have been ident ified by painstakingly obtaining many spectra and finding associated gala-'{ies, however, much larger samples could be found if we had a reliable method of finding groups using photometry. We follow a variation of the Probability Friends of Friends algorithm suggested by Li & Vee (2008) to find galaxy groups in photometric redshift data. Our approach employs a continuous friends-of-friends search in t he t ransverse direction and uses the redshift probability density in redshift space. We use t his method to find galaxy groups in the 4 Canada-France-Hawaii Telescope Legacy Survey Deep Fields, and compare it to a 5th Nearest Neighbor galaxy density search algorithm applied to the same four fields. To fur ther assess our Probability Friends of Friends method, we test it on light cones from the Millennium Simulation. We found that less t han 20% of the Probability Friends of Friends groups with 12 or more members were interlopers. The average density of groups of 12 or more members is ~ 54 per square degree. Our final sample of groups in t he Canada-France-Hawaii Telescope Legacy Survey Deep Fields contains 33 ,279 groups with three or more members, 218 of which have 12 or more members.</p>