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The many flavours of photometric redshifts

Abstract

Since more than 70 years ago, the colours of galaxies derived from flux measurements at different wavelengths have been used to estimate their cosmological distances. Such distance measurements, called photometric redshifts, are necessary for many scientific projects, ranging from investigations of the formation and evolution of galaxies and active galactic nuclei to precision cosmology. The primary benefit of photometric redshifts is that distance estimates can be obtained relatively cheaply for all sources detected in photometric images. The drawback is that these cheap estimates have low precision compared with resource-expensive spectroscopic ones. The methodology for estimating redshifts has been through several revolutions in recent decades, triggered by increasingly stringent requirements on the photometric redshift accuracy. Here, we review the various techniques for obtaining photometric redshifts, from template-fitting to machine learning and hybrid schemes. We also describe state-of-the-art results on current extragalactic samples and explain how survey strategy choices affect redshift accuracy. We close with a description of the photometric redshift efforts planned for upcoming wide-field surveys, which will collect data on billions of galaxies, aiming to investigate, among other matters, the stellar mass assembly and the nature of dark energy.

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Fig. 1: The colour–redshift mapping principle.
Fig. 2: Photometric redshift technique in a nutshell.
Fig. 3: Accuracy of various photo-z surveys as a function of their depth and number of bands.
Fig. 4: Impact of multi-wavelength coverage and choice of templates.
Fig. 5: Cosmological parameter biases incurred by photo-z inaccuracy.

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Acknowledgements

O.I. acknowledges funding of the French Agence Nationale de la Recherche for the SAGACE project, as well as the financial support received from the Centre National d?Etudes Spatiales for the COSMOS project. We thank L.-T. Hsu, J. Chavez-Montego, P. Gonzalez, H. Hildebrandt, M. Brescia and S. Cavuoti for providing the CANDELS/CDFS, ALHAMBRA, SHARDS, KiDS_BPT and KiDS_MLPQN data that were used in Fig. 3.

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Correspondence to Mara Salvato, Olivier Ilbert or Ben Hoyle.

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Salvato, M., Ilbert, O. & Hoyle, B. The many flavours of photometric redshifts. Nat Astron 3, 212–222 (2019). https://doi.org/10.1038/s41550-018-0478-0

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