Euclid: Photometric redshift calibration with self-organising maps
Abstract
The Euclid survey aims to trace the evolution of cosmic structures up to redshift $z$ $\sim$ 3 and beyond. Its success depends critically on obtaining highly accurate mean redshifts for ensembles of galaxies $n(z)$ in all tomographic bins, essential for deriving robust cosmological constraints. However, photometric redshifts (photo-$z$s) suffer from systematic biases arising from various sources of uncertainty. To address these challenges, we utilised self-organising maps (SOMs) with mock samples resembling the Euclid Wide Survey (EWS), to validate Euclid's uncertainty requirement of $|Δ\langle z \rangle| = \langle z_{\text{est}} \rangle - \langle z \rangle \leq 0.002 (1+z)$ per tomographic bin, assuming DR3-level data. We observe that defining the redshift tomography using the mean spectroscopic redshift (spec-$z$) per SOM cell, results in none of the ten tomographic redshift bins satisfying the requirement. In contrast, the redshift tomography on the photo-$z$s of the EWS-like sample yields superior results, with eight out of ten bins [$0 < z\leq 2.5$] meeting the Euclid requirement. To enhance the realism of our study, we morph our calibration sample to mimic the C3R2 survey in incremental steps. In this context, a maximum of six out of ten bins meet the requirement, strongly advocating the adoption of a redshift tomography defined by the photo-$z$s of individual galaxies rather than the commonly used mean spec-$z$ of SOM cells. To examine the impact on the expected biases for $Ω_{\text{m}}$, $σ_{8}$, and $Δw_{0}$ measured by Euclid, we perform a Fisher forecast for cosmic shear only, based on our redshift uncertainties. Here, we find that even under an evaluation of the uncertainty where the impact of the redshift bias is substantial, most absolute biases remain below 0.1$σ$ in the idealised scenario and below 0.3$σ$ in the more realistic case.
Authors 92
-
Laboratoire d’Astrophysique de Marseille
Affiliation as printed
LAM - Laboratoire d'Astrophysique de Marseille (Pôle de l'Étoile Site de Château-Gombert 38, rue Frédéric Joliot-Curie 13388 Marseille cedex 13 - France)
-
Affiliation as printed
ESA - Agence Spatiale Européenne = European Space Agency (Agence spatiale européenne (ESA) 8-10, rue Mario Nikis F-75015 Paris - France)
-
Santiago Casas Aachen
Affiliation as printed
RWTH Aachen
-
Institut de Physique des 2 Infinis de Lyon
Affiliation as printed
IP2I Lyon - Institut de Physique des 2 Infinis de Lyon (Campus LyonTech-la Doua Bâtiment Paul DIRAC 4 Rue Enrico Fermi 69622 Villeurbanne Cedex - France)
-
Laboratoire d’Astrophysique de Marseille
Affiliation as printed
LAM - Laboratoire d'Astrophysique de Marseille (Pôle de l'Étoile Site de Château-Gombert 38, rue Frédéric Joliot-Curie 13388 Marseille cedex 13 - France)
-
Institut de Physique des 2 Infinis de Lyon
Affiliation as printed
IP2I Lyon - Institut de Physique des 2 Infinis de Lyon (Campus LyonTech-la Doua Bâtiment Paul DIRAC 4 Rue Enrico Fermi 69622 Villeurbanne Cedex - France)
-
Centre de physique des particules de Marseille
Affiliation as printed
CPPM - Centre de Physique des Particules de Marseille (163, avenue de Luminy - Case 902 - 13288 Marseille cedex 09 - France)
-
Commissariat à l'Énergie Atomique et aux Énergies Alternatives · CEA Paris-Saclay - Etablissement de Saclay · Astrophysique, Instrumentation et Modélisation · CEA Paris-Saclay
Affiliation as printed
AIM (UMR_7158 / UMR_E_9005 / UM_112) - Astrophysique Interprétation Modélisation (AIM - UMR 7158 - UMR E 9005, ex 'Astrophysique Interactions Multi-échelles', Service d'astrophysique, CEA/Saclay, Orme des Merisiers F-91191 GIF SUR YVETTE CEDEX - France)
-
Institut de Physique des 2 Infinis de Lyon
Affiliation as printed
IP2I Lyon - Institut de Physique des 2 Infinis de Lyon (Campus LyonTech-la Doua Bâtiment Paul DIRAC 4 Rue Enrico Fermi 69622 Villeurbanne Cedex - France)
-
Centre de physique des particules de Marseille
Affiliation as printed
CPPM - Centre de Physique des Particules de Marseille (163, avenue de Luminy - Case 902 - 13288 Marseille cedex 09 - France)
-
Centre de physique des particules de Marseille
Affiliation as printed
CPPM - Centre de Physique des Particules de Marseille (163, avenue de Luminy - Case 902 - 13288 Marseille cedex 09 - France)
-
Institut d'Astrophysique de Paris
Affiliation as printed
IAP - Institut d'Astrophysique de Paris (98bis, bd Arago - 75014 Paris France - France)
-
Centre de physique des particules de Marseille
Affiliation as printed
CPPM - Centre de Physique des Particules de Marseille (163, avenue de Luminy - Case 902 - 13288 Marseille cedex 09 - France)
-
Institut de Physique des 2 Infinis de Lyon
Affiliation as printed
IP2I Lyon - Institut de Physique des 2 Infinis de Lyon (Campus LyonTech-la Doua Bâtiment Paul DIRAC 4 Rue Enrico Fermi 69622 Villeurbanne Cedex - France)
-
Laboratoire d'Étude de l'Univers et des Phénomènes Extrêmes
Affiliation as printed
LUX - Laboratoire d’étude de l’Univers et des phénomènes eXtrèmes (Site de Meudon : 5, place Jules Janssen 92195 MEUDON. Site de Paris : 61, av. de l’Observatoire 75014 PARIS. - France)
-
Laboratoire d’Astrophysique de Marseille
Affiliation as printed
LAM - Laboratoire d'Astrophysique de Marseille (Pôle de l'Étoile Site de Château-Gombert 38, rue Frédéric Joliot-Curie 13388 Marseille cedex 13 - France)
Cited by 0 stored of 0
No patents citing this paper on Lens.org (checked 2026-10-06).