<i>Euclid</i> : Photometric redshift calibration with self-organising maps

The Euclid large-scale weak-lensing survey aims to trace the evolution of cosmic structures up to redshift z ∼ 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) are affected by systematic biases, arising from various sources of uncertainty and dominated by selection effects of the spectroscopic sample used for calibration. To address these challenges, we utilised self-organising maps (SOMs) with mock samples resembling the Euclid Wide Survey (EWS) from the Flagship2 simulation, to validate Euclid ’s uncertainty requirement of |Δ⟨ z ⟩| = ⟨ z est ⟩−⟨ z ⟩≤0.002(1 + z ) per tomographic bin, assuming DR3-level data. Consequently, we identify the most effective galaxy selection for our tomographic bins, while systematically examining the implementation of quality control cuts to reduce sources of uncertainty. In particular, 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 &lt; z ≤ 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 m , σ 8 , and Δ w 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.

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Journal
Astronomy and Astrophysics
Published
2025-12-12
DOI
https://doi.org/10.1051/0004-6361/202556848
Primary Topic
Galaxies: Formation, Evolution, Phenomena
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article
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article

Euclid : Photometric redshift calibration with self-organising maps

C. Baccigalupi, E. Sefusatti, G. Seidel, F.J Castander et al.
Astronomy and Astrophysics
Galaxies: Formation, Evolution, Phenomena
article

Euclid : Photometric redshift calibration with self-organising maps

C. Baccigalupi, E. Sefusatti, G. Seidel, F.J Castander, M. Martinelli, V. Lindholm, P. Tallada-Crespí, A Balestra, L. Moscardini, D. Le Mignant, M. Schirmer, C. Sirignano, M. Brescia, G. Sirri, C. Padilla, S. Farrens, G. Meylan, R Nakajima, J. Carretero, M. Baldi, F. Marulli, X. Dupac, W Gillard, F. Courbin, C. Valieri, I. Lloro, Y. Copin, J. Rhodes, F. Torradeflot, Angus H. Wright, F Faustini, A. Cimatti, J Steinwagner, V. Capobianco, T. Schrabback, L. Gabarra, E. Zucca, S. V. H. Haugan, R. Massey, T. Vassallo, G. Castignani, R. Toledo-Moreo, P. Schneider, M. Moresco, C. Giocoli, J. Skottfelt, D. Sapone, M. Meneghetti, S. Pires, N. Martinet, D. Bonino, W d'Assignies, L. Valenziano, W. Holmes, S Toft, N Tessore, H. Hildebrandt, E. Romelli, Christopher J. Conselice, A. Biviano, E Medinaceli, I. Tutusaus, H. Degaudenzi, O. Mansutti, G. Congedo, L.A Popa, B. Joachimi, J. Väliviita, S. Escoffier, S. Casas, K. George, E. Maiorano, B. Gillis, S. Dusini, M. Roncarelli, Marc Manera, A. Hornstrup, O. Ilbert, F. M. Zerbi, K. Jahnkę, V. Pettorino, B. Altieri, A. Secroun, M. Farina, M Jhabvala, F. Pasian, F. Hormuth, J. Weller, S. Ligori, S. Paltani, R. Farinelli, K. Pedersen, S. Galeotta, E. Franceschi, F. Dubath, Daniel Masters, L. Stanco, M. Poncet, E. Merlin, M. Scodeggio, Robert Reischke, G Cañas-Herrera, J. Gracia-Carpio, A. Grazian, A Amara, S. Camera, G. Polenta, Salud Serrano, E. Rossetti, K. C. Chambers, F. Finelli⋆, A. Costille, E. Keihänen, B. Sartoris, P. Hudelot, S. Cavuoti, N. Auricchio, M. Frailis, M. Fumana, R. Bender, Yongshui Kang, C. Carbone, H.I Teplitz, S. Andreon, P. B. Lilje, A. Renzi, M Melchior, S. Ferriol, A Veropalumbo, C. Burigana, A.M.C Le Brun, E. Branchini, L. Pozzetti, O. Marggraf, S. Kermiche, M. Castellano, G. De Lucia, F. Grupp, H. M. Courtois, W. Roster, Yun Wang, A. Da Silva, G. Riccio, S.-M. Niemi, C. Porciani, Y. Mellier, I. Tereno, S. de la Torre, P Battaglia, S Bardelli, G. Zamorani, H. Kurki‐Suonio, Z. Sakr, S. Mei, D. Maino, W. G. Hartley, C. Neissner, L. Conversi, F. Raison, P. Fosalba, B. Kubik, M. Bolzonella, Y. Ascasíbar, R. Saglia, M. Sereno, V. F. Cardone, R. Casas, N. Fourmanoit, A. N. Taylor, A. Mora, C. Rosset, H. Hoekstra, C. A. J. Duncan, R. Rebolo
article en

Abstract

The Euclid large-scale weak-lensing survey aims to trace the evolution of cosmic structures up to redshift z ∼ 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) are affected by systematic biases, arising from various sources of uncertainty and dominated by selection effects of the spectroscopic sample used for calibration. To address these challenges, we utilised self-organising maps (SOMs) with mock samples resembling the Euclid Wide Survey (EWS) from the Flagship2 simulation, to validate Euclid ’s uncertainty requirement of |Δ⟨ z ⟩| = ⟨ z est ⟩−⟨ z ⟩≤0.002(1 + z ) per tomographic bin, assuming DR3-level data. Consequently, we identify the most effective galaxy selection for our tomographic bins, while systematically examining the implementation of quality control cuts to reduce sources of uncertainty. In particular, 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 &lt; z ≤ 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 m , σ 8 , and Δ w 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.

Astronomy and AstrophysicsVol. 707
Université Claude Bernard Lyon 1 (FR), University of Geneva (CH), Institució Catalana de Recerca i Estudis Avançats (ES), Leiden University (NL), California Institute of Technology (US), Universitat Autònoma de Barcelona (ES), University of Copenhagen (DK), Centre National de la Recherche Scientifique (FR), Goddard Space Flight Center (US), University of Hawaii System (US), Jet Propulsion Laboratory (US), Université Toulouse III - Paul Sabatier (FR), Consejo Superior de Investigaciones Científicas (ES), University of Bonn (DE), Scuola Internazionale Superiore di Studi Avanzati (IT), University of Padua (IT), University of Lisbon (PT), Universidad de La Laguna (ES), University of Groningen (NL), Université Fédérale de Toulouse Midi-Pyrénées (FR), Lyon College (US), Université Catholique de Lille (FR), University of Oslo (NO), Institute for High Energy Physics (ES), University of Milan (IT), Universität Innsbruck (AT), University of Saint Joseph (US), Université Paris Cité (FR), Aix-Marseille Université (FR), Heidelberg University (DE), Commissariat à l'Énergie Atomique et aux Énergies Alternatives (FR), Université Paris Sciences et Lettres (FR), Université Paris-Saclay (FR), Centre National d'Études Spatiales (FR), Helsinki Institute of Physics (FI), Astronomical Observatory of Rome (IT), Trieste Astronomical Observatory (IT), Institut d'Astrophysique de Paris (FR), Astronomical Observatory of Capodimonte (IT), European Space Agency (FR), Laboratoire AstroParticule et Cosmologie (FR), Brera Astronomical Observatory (IT), University of Surrey (GB), FHNW University of Applied Sciences and Arts Northwestern Switzerland (CH), United Kingdom Space Agency (GB), Universidad Politécnica de Cartagena (ES), Sorbonne Université (FR), University of Oxford (GB), Park University (US), Osservatorio astronomico di Bologna (IT), Astrophysique, Instrumentation et Modélisation (FR), Institute of Astrophysics and Space Sciences (PT), Max Planck Institute for Extraterrestrial Physics (DE), Institut d'Estudis Espacials de Catalunya (ES), Istituto Nazionale di Fisica Nucleare, Sezione di Torino (IT), Institute for Space Astrophysics and Planetology (IT), International University of Beirut (LB), Institut Català de Ciències del Clima (ES), Istituto Nazionale di Fisica Nucleare, Sezione di Padova (IT), Osservatorio Astrofisico di Torino (IT), Royal Observatory (GB), SKA Observatory (GB), Istituto Nazionale di Fisica Nucleare, Sezione di Napoli (IT), Max Planck Institute for Astronomy (DE), Institute of Space Sciences (ES), DAWN Center for Independent Living (US), Istituto di Radioastronomia di Bologna (IT), Federico II University Hospital (IT), Unidades Centrales Científico-Técnicas (ES), Institute of Theoretical Physics (CN), Istituto Nazionale di Fisica Nucleare, Sezione di Genova (IT), CEA Paris-Saclay (FR), Naples Anesthesia & Physician Associates (US), Instituto de Astrofísica de Canarias (ES), Felix Scholarship (GB), Astronomical Institute of the Slovak Academy of Sciences (SK), Istituto Nazionale di Fisica Nucleare, Sezione di Milano (IT), Institute of Space Science - INFLPR Subsidiary (RO), Engineering Science Analysis (United States) (US), Istituto Nazionale di Fisica Nucleare, Sezione di Bologna (IT), Port d'Informació Científica (ES), Istituto Nazionale di Fisica Nucleare, Sezione di Trieste (IT), Institute for Fundamental Physics of the Universe (IT), Astronomical Observatory (RS), St. Joseph Medical Center (US), Istituto Nazionale di Fisica Nucleare, Sezione di Roma I (IT), Osservatorio Astronomico di Padova (IT), European Space Research Institute (IT), Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (ES), Institut de Recherche en Astrophysique et Planétologie (FR), Infrared Processing and Analysis Center (US), Osservatorio di Astrofisica e Scienza dello Spazio, European Space Research and Technology Centre (NL), University College London (GB), École Polytechnique Fédérale de Lausanne (CH), University of Turin (IT), University of Chile (CL), Heidelberg Institute for Theoretical Studies (DE), Open University Malaysia (MY), Ludwig-Maximilians-Universität München (DE), University of Genoa (IT), National Institute for Astrophysics (IT), RWTH Aachen University (DE), University of Bologna (IT), Technical University of Denmark (DK), University of Edinburgh (GB)
Openalex Percentile: Top 72%
Galaxies: Formation, Evolution, Phenomena
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