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 °^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 σΔ z/(1+z)=0.029 with η=3.8% of catastrophic errors (η is defined strictly as those objects with |Δ z|/(1+z) > 0.15). The reliability of our photometric redshifts decreases for faint objects: we find σΔ z/(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 deg^2. These photometric redshifts are made publicly available at http://terapix.iap.fr (complete ascii catalogues) and http://cencos.oamp.fr/cencos/CFHTLS/ (searchable database interface).
Accurate photometric redshifts for the CFHT legacy survey calibrated using the VIMOS VLT deep survey / Ilbert O.; Arnouts S.; McCracken H. J.; Bolzonella M.; Bertin E.; Le Fèvre O.; Mellier Y.; Zamorani G.; Pellò R.; Iovino A.; Tresse L.; Le Brun V.; Bottini D.; Garilli B.; Maccagni D.; Picat J. P.; Scaramella R.; Scodeggio M.; Vettolani G.; Zanichelli A.; Adami C.; Bardelli S.; Cappi A.; Charlot S.; Ciliegi P.; Contini T.; Cucciati O.; Foucaud S.; Franzetti P.; Gavignaud I.; Guzzo L.; Marano B.; Marinoni C.; Mazure A.; Meneux B.; Merighi R.; Paltani S.; Pollo A.; Pozzetti L.; Radovich M.; Zucca E.; Bondi M.; Bongiorno A.; Busarello G.; de La Torre S.; Gregorini L.; Lamareille F.; Mathez G.; Merluzzi P.; Ripepi V.; Rizzo D.; Vergani D.. - In: ASTRONOMY & ASTROPHYSICS. - ISSN 0004-6361. - STAMPA. - 457:(2006), pp. 841-856. [10.1051/0004-6361:20065138]
Accurate photometric redshifts for the CFHT legacy survey calibrated using the VIMOS VLT deep survey
CUCCIATI, OLGA;MARANO, BRUNO;BONGIORNO, ANGELA;GREGORINI, LORETTA;
2006
Abstract
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 °^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 σΔ z/(1+z)=0.029 with η=3.8% of catastrophic errors (η is defined strictly as those objects with |Δ z|/(1+z) > 0.15). The reliability of our photometric redshifts decreases for faint objects: we find σΔ z/(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 deg^2. These photometric redshifts are made publicly available at http://terapix.iap.fr (complete ascii catalogues) and http://cencos.oamp.fr/cencos/CFHTLS/ (searchable database interface).I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
