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Astron. Astrophys. 359, 9-17 (2000)

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Quasar candidate multicolor selection technique: a different approach

E. Hatziminaoglou, G. Mathez and R. Pelló

Observatoire Midi-Pyrénées, Laboratoire d'Astrophysique, UMR 5572, 14 Avenue E. Belin, 31400 Toulouse, France

Received 1 February 2000 / Accepted 28 April 2000


We present a quasar candidate identification technique based on multicolor photometry. The traditional multi-dimensional method (2 [FORMULA] N dimensions, where N is the number of the color-color diagrams) is reduced to a one-dimensional technique, which consists in a standard fitting procedure, where the observed spectral energy distributions are compared to quasar simulated spectra and stellar templates. This new multicolor approach is firstly applied to simulated catalogues and its efficiency is examined in various redshift ranges, as a function of the filter combination and the available observing time for spectroscopy. We conclude that this method is better suited than the usual multicolor selection techniques to quasar identification, especially for high-redshift quasars. The application of the method to real quasar samples found in the literature results in an efficiency comparable to the one obtained from the use of color-color diagrams. The major advantage of the new method is the estimation of the photometric redshift of quasar candidates, enabling, in almost all cases, spectroscopy to be targeted to best suited wavelength ranges.

Key words: galaxies: photometry – galaxies: quasars: general – cosmology: miscellaneous

Send offprint requests to: E. Hatziminaoglou (eva@ast.obs-mip.fr)

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© European Southern Observatory (ESO) 2000

Online publication: June 30, 2000