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The PAU Survey: an improved photo-z sample in the COSMOS field

Alarcon, Alex and Gaztanaga, Enrique and Eriksen, Martin and Baugh, Carlton M and Cabayol, Laura and Casas, Ricard and Carretero, Jorge and Castander, Francisco J and De Vicente, Juan and Fernandez, Enrique and Garcia-Bellido, Juan and Hildebrandt, Hendrik and Hoekstra, Henk and Joachimi, Benjamin and Manzoni, Giorgio and Miquel, Ramon and Norberg, Peder and Padilla, Cristobal and Renard, Pablo and Sanchez, Eusebio and Serrano, Santiago and Sevilla-Noarbe, Ignacio and Siudek, Malgorzata and Tallada-Crespí, Pau (2021) 'The PAU Survey: an improved photo-z sample in the COSMOS field.', Monthly notices of the Royal Astronomical Society, 501 (4). pp. 6103-6122.


We present – and make publicly available – accurate and precise photometric redshifts in the ACS footprint from the COSMOS field for objects with iAB ≤ 23. The redshifts are computed using a combination of narrow-band photometry from PAUS, a survey with 40 narrow bands spaced at 100A˚ intervals covering the range from 4500 to 8500A˚⁠, and 26 broad, intermediate, and narrow bands covering the UV, visible and near-infrared spectrum from the COSMOS2015 catalogue. We introduce a new method that models the spectral energy distributions as a linear combination of continuum and emission-line templates and computes its Bayes evidence, integrating over the linear combinations. The correlation between the UV luminosity and the O II line is measured using the 66 available bands with the zCOSMOS spectroscopic sample, and used as a prior which constrains the relative flux between continuum and emission-line templates. The flux ratios between the O II line and Hα, Hβ and OIII are similarly measured and used to generate the emission-line templates. Comparing to public spectroscopic surveys via the quantity Δz ≡ (zphoto − zspec)/(1 + zspec), we find the photometric redshifts to be more precise than previous estimates, with σ68(Δz) ≈ (0.003, 0.009) for galaxies at magnitude iAB ∼ 18 and iAB ∼ 23, respectively, which is three times and 1.66 times tighter than COSMOS2015. Additionally, we find the redshifts to be very accurate on average, yielding a median of the Δz distribution compatible with |median(Δz)| ≤ 0.001 at all redshifts and magnitudes considered. Both the added PAUS data and new methodology contribute significantly to the improved results. The catalogue produced with the technique presented here is expected to provide a robust redshift calibration for current and future lensing surveys, and allows one to probe galaxy formation physics in an unexplored luminosity-redshift regime, thanks to its combination of depth, completeness, and excellent redshift precision and accuracy.

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Publisher statement:This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2021 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Date accepted:17 November 2020
Date deposited:01 July 2021
Date of first online publication:22 January 2021
Date first made open access:01 July 2021

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