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Published January 10, 2009 | Published
Journal Article Open

Cosmos Photometric Redshifts with 30-Bands for 2-deg^2


We present accurate photometric redshifts (photo-z) in the 2-deg^2 COSMOS field. The redshifts are computed with 30 broad, intermediate, and narrowbands covering the UV (Galaxy Evolution Explorer), visible near-IR (NIR; Subaru, Canada-France-Hawaii Telescope (CFHT), United Kingdom Infrared Telescope, and National Optical Astronomy Observatory), and mid-IR (Spitzer/IRAC). A χ^2 template-fitting method (Le Phare) was used and calibrated with large spectroscopic samples from the Very Large Telescope Visible Multi-Object Spectrograph and the Keck Deep Extragalactic Imaging Multi-Object Spectrograph. We develop and implement a new method which accounts for the contributions from emission lines ([O II], Hβ, Hα, and Lyα) to the spectral energy distributions (SEDs). The treatment of emission lines improves the photo-z accuracy by a factor of 2.5. Comparison of the derived photo-z with 4148 spectroscopic redshifts (i.e., Δz = z_s – z_p) indicates a dispersion of at σΔ_z/(1+z_s) = 0.007 at i^+_(AB) < 22.5, a factor of 2-6 times more accurate than earlier photo-z in the COSMOS, CFHT Legacy Survey, and the Classifying Object by Medium-Band Observations-17 survey fields. At fainter magnitudes i^+_( AB) < 24 and z < 1.25, the accuracy is σΔ_z/(1+z_s) = 0.012. The deep NIR and Infrared Array Camera coverage enables the photo-z to be extended to z ~ 2, albeit with a lower accuracy (σΔ_z/(1+z_s) = 0.06 at i^+_(AB) ~ 24). The redshift distribution of large magnitude-selected samples is derived and the median redshift is found to range from z m = 0.66 at 22 < i^+_(AB) < 22.5 to z_m = 1.06 at 24.5 < i^+_(AB) < 25. At i^+_(AB) < 26.0, the multiwavelength COSMOS catalog includes approximately 607,617 objects. The COSMOS-30 photo-z enables the full exploitation of this survey for studies of galaxy and large-scale structure evolution at high redshift.

Additional Information

© 2009 The American Astronomical Society. Received 2008 August 15, accepted for publication 2008 September 10. Published 2008 December 8. We gratefully acknowledge the contributions of the entire COSMOS collaboration consisting of more than 100 scientists. The HST COSMOS program was supported through NASA grant HST-GO-09822. More information on the COSMOS survey is available at Web site http://www.astro.caltech.edu/cosmos. We also greatly appreciate the hospitality provided by the Aspen Center for Physics where this manuscript was completed. This article 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. It is also based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under NASA contract 1407. The research is also based on data collected at: the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan; the XMM-Newton, an ESAscience mission with instruments and contributions directly funded by ESAMember 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 (CFHT) with MegaPrime/MegaCam operated as a joint project by the CFHT Corporation, CEA/DAPNIA, the NRC and CADC of Canada, the CNRS of France, TERAPIX, and the University of Hawaii.

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