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Published December 1, 2011 | Published
Journal Article Open

Galaxies in X-Ray Groups. I. Robust Membership Assignment and the Impact of Group Environments on Quenching


Understanding the mechanisms that lead dense environments to host galaxies with redder colors, more spheroidal morphologies, and lower star formation rates than field populations remains an important problem. As most candidate processes ultimately depend on host halo mass, accurate characterizations of the local environment, ideally tied to halo mass estimates and spanning a range in halo mass and redshift, are needed. In this work, we present and test a rigorous, probabilistic method for assigning galaxies to groups based on precise photometric redshifts and X-ray-selected groups drawn from the COSMOS field. The groups have masses in the range 10^(13) ≾ M_(200c)/M_☉ ≾ 10^(14) and span redshifts 0 < z < 1. We characterize our selection algorithm via tests on spectroscopic subsamples, including new data obtained at the Very Large Telescope, and by applying our method to detailed mock catalogs. We find that our group member galaxy sample has a purity of 84% and completeness of 92% within 0.5R_(200c). We measure the impact of uncertainties in redshifts and group centering on the quality of the member selection with simulations based on current data as well as future imaging and spectroscopic surveys. As a first application of our new group member catalog which will be made publicly available, we show that member galaxies exhibit a higher quenched fraction compared to the field at fixed stellar mass out to z ~ 1, indicating a significant relationship between star formation and environment at group scales. We also address the suggestion that dusty star-forming galaxies in such groups may impact the high-ℓ power spectrum of the cosmic microwave background and find that such a population cannot explain the low power seen in recent Sunyaev-Zel'dovich measurements.

Additional Information

© 2011 The American Astronomical Society. Received 2011 June 6; accepted 2011 September 23; published 2011 November 15. We thank Joanne Cohn, Eliot Quataert, Eli Rykoff, David Schlegel, Uros Seljak, Erik Shirokoff, Andrew Wetzel, and Martin White for helpful conversations. We also thank Michael Cooper and Marc Davis for providing software and template spectra used in our spectroscopic analysis, as well as comments on the paper. M.R.G. is supported by a Graduate Research Fellowship from the National Science Foundation. This work is partly based on observations made with ESO Telescopes at Paranal Observatory under program ID 084.B- 0523.We gratefully acknowledge the contributions of the entire COSMOS collaboration consisting of more than 70 scientists. More information on the COSMOS survey is available at http://www.astro.caltech.edu/cosmos. This work is based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA Inc., under NASA contract NAS 5-26555; also based on data collected at: the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan; the XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA; the European Southern Observatory under Large Program 175.A-0839, Chile; Kitt Peak National Observatory, Cerro Tololo Inter-American Observatory, and the National Optical Astronomy Observatory, which are operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation; and the Canada–France–Hawaii Telescope with MegaPrime/MegaCam operated as a joint project by the CFHT Corporation, CEA/DAPNIA, the National Research Council of Canada, the Canadian Astronomy Data Centre, the Centre National de la Recherche Scientifique de France, TERAPIX, and the University of Hawaii.

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