Accurate photometric redshifts for the CFHT legacy survey calibrated using the VIMOS VLT deep survey

O. Ilbert, S. Arnouts, H. J. McCracken, M. Bolzonella, E. Bertin, O. Le Fèvre, Y. Mellier, G. Zamorani, R. Pellò, A. Iovino, L. Tresse, V. Le Brun, D. Bottini, B. Garilli, D. Maccagni, J. P. Picat, R. Scaramella, M. Scodeggio, G. Vettolani, A. ZanichelliC. Adami, S. Bardelli, A. Cappi, S. Charlot, P. Ciliegi, T. Contini, O. Cucciati, S. Foucaud, P. Franzetti, I. Gavignaud, L. Guzzo, B. Marano, C. Marinoni, A. Mazure, B. Meneux, R. Merighi, S. Paltani, A. Pollo, L. Pozzetti, M. Radovich, E. Zucca, M. Bondi, A. Bongiorno, G. Busarello, S. De La Torre, L. Gregorini, F. Lamareille, G. Mathez, P. Merluzzi, V. Ripepi, D. Rizzo, D. Vergani

Research output: Contribution to journalReview articlepeer-review

1228 Citations (Scopus)


Aims. We present and release photometric redshifts for a uniquely large and deep sample of 522286 objects with i′AB ≤25 in the Canada-France Hawaii Telescope Legacy Survey (CFHTLS) "Deep Survey" fields D1, D2, D3, and D4, which cover a total effective area of 3.2 deg 2. Methods. We use 3241 spectroscopic redshifts with 0 ≤ z ≤ 5 from the VIMOS VLT Deep Survey (VVDS) as a calibration and training set to derive these photometric redshifts. Using the "Le Phare" photometric redshift code, we developed a robust calibration method based on an iterative zero-point refinement combined with a template optimisation procedure and the application of a Bayesian approach. This method removes systematic trends in the photometric redshifts and significantly reduces the fraction of catastrophic errors (by a factor of 2), a significant improvement over traditional methods. We use our unique spectroscopic sample to present a detailed assessment of the robustness of the photometric redshift sample. Results. For a sample selected at i′AB ≤24, we reach a redshift accuracy of σΔz/(1+z)= 0.029 with η = 3.8% of catastrophic errors (η is defined strictly as those objects with |Δz|/(1 + z) > 0.15). The reliability of our photometric redshifts decreases for faint objects: we find σΔz/(1+z) = 0.025,0.034 and η = 1.9%, 5.5% for samples selected at i′AB = 17.5-22.5 and 22.5-24 respectively. We find that the photometric redshifts of starburst galaxies are less reliable: although these galaxies represent only 22% of the spectroscopic sample, they are responsible for 50% of the catastrophic errors. An analysis as a function of redshift demonstrates that our photometric redshifts work best in the redshift range 0.2 ≤; z ≤ 1.5. We find an excellent agreement between the photometric and the VVDS spectroscopic redshift distributions at i′AB ≤ 24. Finally, we compare the redshift distributions of i′ selected galaxies on the four CFHTLS deep fields, showing that cosmic variance is still present on fields of 0.7-0.9 deg2. These photometric redshifts are made publicly available at (complete ascii catalogues) and (searchable database interface).

Original languageEnglish
Pages (from-to)841-856
Number of pages16
JournalAstronomy and Astrophysics
Issue number3
Publication statusPublished - Oct 2006


  • Galaxies: distances and redshifts
  • Galaxies: photometry
  • Methods: data analysis

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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