Euclid preparation. LXXXIII. The impact of redshift interlopers on the two-point correlation function analysis

Euclid Collaboration; Risso, I.; Veropalumbo, A.; Branchini, E.; Maragliano, E.; de la Torre, S.; Sarpa, E.; Monaco, P.; Granett, B. R.; Lee, S.; Addison, G. E.; Bruton, S.; Carbone, C.; Lavaux, G.; Markovic, K.; McCarthy, K.; Parimbelli, G.; Passalacqua, F.; Percival, W. J.; Scarlata, C.; Sefusatti, E.; Wang, Y.; Bonici, M.; Oppizzi, F.; Aghanim, N.; Altieri, B.; Amara, A.; Andreon, S.; Auricchio, N.; Baccigalupi, C.; Baldi, M.; Balestra, A.; Bardelli, S.; Battaglia, P.; Biviano, A.; Bonchi, A.; Bonino, D.; Brescia, M.; Brinchmann, J.; Camera, S.; Cañas-Herrera, G.; Capobianco, V.; Cardone, V. F.; Carretero, J.; Casas, S.; Castellano, M.; Castignani, G.; Cavuoti, S.; Chambers, K. C.; Cimatti, A.; Colodro-Conde, C.; Congedo, G.; Conselice, C. J.; Conversi, L.; Copin, Y.; Courbin, F.; Courtois, H. M.; Crocce, M.; Da Silva, A.; Degaudenzi, H.; De Lucia, G.; Di Giorgio, A. M.; Dole, H.; Douspis, M.; Dubath, F.; Duncan, C. A. J.; Dupac, X.; Dusini, S.; Escoffier, S.; Farina, M.; Farinelli, R.; Faustini, F.; Ferriol, S.; Finelli, F.; Fotopoulou, S.; Fourmanoit, N.; Frailis, M.; Franceschi, E.; Fumana, M.; Galeotta, S.; George, K.; Gillard, W.; Gillis, B.; Giocoli, C.; Gracia-Carpio, J.; Grazian, A.; Grupp, F.; Guzzo, L.; Haugan, S. V. H.; Holmes, W.; Hormuth, F.; Hornstrup, A.; Hudelot, P.; Jahnke, K.; Jhabvala, M.; Joachimi, B.; Keihänen, E.; Kermiche, S.; Kiessling, A.; Kilbinger, M.; Kubik, B.; Kümmel, M.; Kunz, M.; Kurki-Suonio, H.; Le Brun, A. M. C.; Liebing, P.; Ligori, S.; Lilje, P. B.; Lindholm, V.; Lloro, I.; Mainetti, G.; Maino, D.; Maiorano, E.; Mansutti, O.; Marcin, S.; Marggraf, O.; Martinelli, M.; Martinet, N.; Marulli, F.; Massey, R.; Maurogordato, S.; Medinaceli, E.; Mei, S.; Melchior, M.; Mellier, Y.; Meneghetti, M.; Merlin, E.; Meylan, G.; Mora, A.; Moresco, M.; Moscardini, L.; Nakajima, R.; Neissner, C.; Niemi, S.-M.; Nightingale, J. W.; Padilla, C.; Paltani, S.; Pasian, F.; Pedersen, K.; Pettorino, V.; Pires, S.; Polenta, G.; Poncet, M.; Popa, L. A.; Pozzetti, L.; Raison, F.; Rebolo, R.; Renzi, A.; Rhodes, J.; Riccio, G.; Romelli, E.; Roncarelli, M.; Rossetti, E.; Saglia, R.; Sakr, Z.; Sapone, D.; Sartoris, B.; Schewtschenko, J. A.; Schneider, P.; Schrabback, T.; Scodeggio, M.; Secroun, A.; Seidel, G.; Seiffert, M.; Serrano, S.; Simon, P.; Sirignano, C.; Sirri, G.; Stanco, L.; Steinwagner, J.; Surace, C.; Tallada-Crespí, P.; Tavagnacco, D.; Taylor, A. N.; Tereno, I.; Tessore, N.; Toft, S.; Toledo-Moreo, R.; Torradeflot, F.; Tutusaus, I.; Valenziano, L.; Valiviita, J.; Vassallo, T.; Verdoes Kleijn, G.; Vibert, D.; Weller, J.; Zamorani, G.; Zerbi, F. M.; Zucca, E.; Allevato, V.; Ballardini, M.; Bolzonella, M.; Bozzo, E.; Burigana, C.; Cabanac, R.; Cappi, A.; Di Ferdinando, D.; Escartin Vigo, J. A.; Gabarra, L.; Hartley, W. G.; Martín-Fleitas, J.; Matthew, S.; Mauri, N.; Metcalf, R. B.; Pezzotta, A.; Pöntinen, M.; Porciani, C.; Scottez, V.; Sereno, M.; Tenti, M.; Viel, M.; Wiesmann, M.; Akrami, Y.; Alvi, S.; Andika, I. T.; Archidiacono, M.; Atrio-Barandela, F.; Avila, S.; Balaguera-Antolinez, A.; Benoist, C.; Bertacca, D.; Bethermin, M.; Blot, L.; Böhringer, H.; Borgani, S.; Brown, M. L.; Calabro, A.; Camacho Quevedo, B.; Caro, F.; Carvalho, C. S.; Castro, T.; Cogato, F.; Cooray, A. R.; Cucciati, O.; Davini, S.; De Paolis, F.; Desprez, G.; Díaz-Sánchez, A.; Diaz, J. J.; Di Domizio, S.; Diego, J. M.; Dimauro, P.; Enia, A.; Fang, Y.; Ferrari, A. G.; Finoguenov, A.; Fontana, A.; Franco, A.; Ganga, K.; García-Bellido, J.; Gasparetto, T.; Gautard, V.; Gaztanaga, E.; Giacomini, F.; Gianotti, F.; Gozaliasl, G.; Guidi, M.; Gutierrez, C. M.; Hall, A.; Hemmati, S.; Hernández-Monteagudo, C.; Hildebrandt, H.; Hjorth, J.; Joudaki, S.; Kajava, J. J. E.; Kang, Y.; Kansal, V.; Karagiannis, D.; Kiiveri, K.; Kirkpatrick, C. C.; Kruk, S.; Le Brun, V.; Le Graet, J.; Legrand, L.; Lembo, M.; Lepori, F.; Leroy, G.; Lesci, G. F.; Leuzzi, L.; Liaudat, T. I.; Loureiro, A.; Macias-Perez, J.; Magliocchetti, M.; Mannucci, F.; Maoli, R.; Martins, C. J. A. P.; Maurin, L.; Miluzio, M.; Moretti, C.; Morgante, G.; Nadathur, S.; Naidoo, K.; Navarro-Alsina, A.; Paterson, K.; Patrizii, L.; Pisani, A.; Potter, D.; Quai, S.; Radovich, M.; Rocci, P.-F.; Sacquegna, S.; Sahlén, M.; Sanders, D. B.; Schneider, A.; Sciotti, D.; Sellentin, E.; Smith, L. C.; Sorce, J. G.; Tanidis, K.; Tao, C.; Testera, G.; Teyssier, R.; Tosi, S.; Troja, A.; Tucci, M.; Valieri, C.; Venhola, A.; Vergani, D.; Verza, G.; Walton, N. A.

doi:10.1051/0004-6361/202555402

Published March 2026 | Version Published

Journal Article Open

Euclid preparation. LXXXIII. The impact of redshift interlopers on the two-point correlation function analysis

1. Brera Astronomical Observatory
2. INFN Sezione di Genova
3. University of Genoa
4. French National Centre for Scientific Research
5. International School for Advanced Studies
6. ICSC – Centro Nazionale di Ricerca in High Performance Computing, Big Data e Quantum Computing, Via Magnanelli 2, Bologna, Italy
7. INFN Sezione di Trieste
8. University of Trieste
9. Trieste Astronomical Observatory
10. Institute for Fundamental Physics of the Universe
11. Jet Propulsion Lab
12. Johns Hopkins University
13. California Institute of Technology
14. Istituto di Astrofisica Spaziale e Fisica Cosmica di Milano
15. Institut d'Astrophysique de Paris
16. Institute of Space Sciences
17. University of Padua
18. INFN Sezione di Padova
19. University of Waterloo
20. Perimeter Institute
21. University of Minnesota
22. Infrared Processing and Analysis Center
23. Institut d'Astrophysique Spatiale
24. European Space Astronomy Centre
25. University of Surrey
26. INAF-Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Piero Gobetti 93/3, 40129, Bologna, Italy
27. University of Bologna
28. INFN Sezione di Bologna
29. Osservatorio Astronomico di Padova
30. Agenzia Spaziale Italiana
31. Osservatorio Astrofisico di Torino
32. University of Naples Federico II
33. Astronomical Observatory of Capodimonte
34. University of Porto
35. University of Turin
36. INFN Sezione di Torino
37. European Space Research and Technology Centre
38. Leiden University
39. Astronomical Observatory of Rome
40. INFN Sezione di Roma I
41. Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas
42. Port d'Informació Científica
43. RWTH Aachen University
44. INFN Sezione di Napoli
45. University of Hawaii at Manoa
46. Instituto de Astrofísica de Canarias
47. University of Edinburgh
48. University of Manchester
49. European Space Research Institute
50. Claude Bernard University Lyon 1
51. University of Barcelona
52. Institució Catalana de Recerca i Estudis Avançats
53. UCB Lyon 1, CNRS/IN2P3, IUF, IP2I Lyon, 4 rue Enrico Fermi, 69622, Villeurbanne, France
54. Institut d'Estudis Espacials de Catalunya
55. University of Lisbon
56. University of Geneva
57. Institute for Space Astrophysics and Planetology
58. Center for Particle Physics of Marseilles
59. University of Bristol
60. Ludwig-Maximilians-Universität München
61. Max Planck Institute for Extraterrestrial Physics
62. University of Milan
63. INFN Sezione di Milano
64. University of Oslo
65. Felix Hormuth Engineering, Goethestr. 17, 69181, Leimen, Germany
66. Technical University of Denmark
67. University of Copenhagen
68. Max Planck Institute for Astronomy
69. Goddard Space Flight Center
70. University College London
71. University of Helsinki
72. University of Paris
73. Helsinki Institute of Physics
74. Sorbonne University
75. Netherlands Institute for Radio Astronomy
76. Centre de Calcul de l'Institut National de Physique Nucléaire et de Physique des Particules
77. University of Applied Sciences and Arts Northwestern Switzerland
78. University of Bonn
79. Durham University
80. Observatoire de la Côte d'Azur
81. Lagrange Laboratory
82. Astroparticle and Cosmology Laboratory
83. CNRS-UCB International Research Laboratory, Centre Pierre Binétruy, IRL2007, CPB-IN2P3, Berkeley, USA
84. École Polytechnique Fédérale de Lausanne
85. Aurora Technology for European Space Agency (ESA), Camino bajo del Castillo s/n Urbanizacion Villafranca del Castillo, Villanueva de la Cañada, 28692, Madrid, Spain
86. Institute for High Energy Physics
87. Newcastle University
88. Centre National d'Études Spatiales
89. Institute of Space Science
90. Spanish National Research Council
91. University of La Laguna
92. Heidelberg University
93. Research Institute in Astrophysics and Planetology
94. Université St Joseph; Faculty of Sciences, Beirut, Lebanon
95. University of Chile
96. Universität Innsbruck
97. Satlantis, University Science Park, Sede Bld, 48940, Leioa-Bilbao, Spain
98. Cosmic Dawn Center (DAWN)
99. Polytechnic University of Cartagena
100. University of Groningen
101. University of Ferrara
102. INFN Sezione di Ferrara
103. Istituto di Radioastronomia di Bologna
104. University of Oxford
105. Groupe Institut supérieur d'agriculture de Lille
106. Institute for Theoretical Physics
107. Case Western Reserve University
108. Technical University of Munich
109. Max Planck Institute for Astrophysics
110. University of Salamanca
111. Observatory of Strasbourg
112. University of Tokyo
113. Max Planck Institute for Physics
114. University of California, Irvine
115. University of Salento
116. INFN Sezione di Lecce
117. INAF-Sezione di Lecce, c/o Dipartimento Matematica e Fisica, Via per Arnesano, 73100, Lecce, Italy
118. Institute of Physics of Cantabria
119. National Observatory
120. CEA Saclay
121. University of Portsmouth
122. Aalto University
123. Ruhr University Bochum
124. University of Turku
125. Serco for European Space Agency (ESA), Camino bajo del Castillo s/n Urbanizacion Villafranca del Castillo, Villanueva de la Cañada, 28692, Madrid, Spain
126. ARC Centre of Excellence for Dark Matter Particle Physics
127. Swinburne University of Technology
128. University of the Western Cape
129. University of Cambridge
130. University of Zurich
131. Institut de Recherche sur les Lois Fondamentales de l'Univers
132. Stockholm University
133. Imperial College London
134. Grenoble Institute of Technology
135. Arcetri Astrophysical Observatory
136. Sapienza University of Rome
137. Centre for Astrophysics of the University of Porto
138. HE Space for European Space Agency (ESA), Camino bajo del Castillo s/n Urbanizacion Villafranca del Castillo, Villanueva de la Cañada, 28692, Madrid, Spain
139. Princeton University
140. Uppsala University
141. University of Lille
142. University of Oulu
143. Center for Computational Astrophysics, Flatiron Institute, 162 5th Avenue, 10010, New York, NY, USA

Context. The Euclid galaxy survey is designed to measure the spectroscopic redshift of emission-line galaxies (ELGs) by identifying the Hα emission line in their slitless spectra. The efficacy of this approach crucially depends on the signal-to-noise ratio (S/N) of the line, as sometimes noise fluctuations in the spectrum continuum can be misidentified as Hα. In addition, other genuine strong emission lines can be mistaken for Hα, depending on the redshift of the source. Both effects lead to ambiguities in the redshift measurement that can result in catastrophic redshift errors and the inclusion of ‘interloper’ galaxies in the sample.

Aims. This paper forecasts the impact on the galaxy clustering analysis of the expected redshift errors in the Euclid spectroscopic sample. Specifically, it investigates the effect of the redshift interloper contamination on the galaxy two-point correlation function (2PCF) and, in turn, on the inferred growth rate of structure fσ₈ and Alcock–Paczynski (AP) parameters α_∥ and α_⊥.

Methods. This work is based on the analysis of 1000 synthetic spectroscopic catalogues, the EuclidLargeMocks, which mimic the area and selection function of the Euclid Data Release 1 (DR1) sample. We estimated the 2PCF of contaminated catalogues and separated the different contributions, particularly those coming from galaxies with correctly measured redshift and from contaminants. We explored different models of increasing complexity to describe the measured 2PCF at a fixed cosmology, with the aim of identifying the most efficient model to reproduce the data. Finally, we performed a cosmological inference and evaluated the systematic error on the inferred fσ₈, α_∥, and α_⊥ values associated with different models.

Results. Our results demonstrate that a minimal modelling approach, which only accounts for an attenuation of the clustering signal regardless of the type of contaminants, is sufficient to recover the correct values of fσ₈, α_∥, and α_⊥ at DR1. The accuracy and precision of the estimated AP parameters are largely insensitive to the presence of interlopers. The adoption of a minimal modelling induces a 1%–3% systematic error on the growth rate of structure estimation, depending on the considered redshift. However, this error remains smaller than the statistical error expected for the Euclid DR1 analysis.

Copyright and License

© The Authors 2026. Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Acknowledgement

The authors acknowledges support from MIUR, PRIN 2022 (grant 2022NY2ZRS 001). Simulations and computations in this work have been run at the computing facilities of INFN, Sezione di Genova: the authors wish to thank the INFN IT personnel in Genova for their precious and constant support. P.M. acknowledges support from Italian Research Center on High Performance Computing Big Data and Quantum Computing (ICSC), by the Fondazione ICSC National Recovery and Resilience Plan (PNRR) Project ID CN-00000013 and by the PRIN 2022 PNRR project (code no. P202259YAF) funded by “European Union – Next Generation EU”, Mission 4, Component 1, CUP J53D23019100001. We acknowledge usage of Pleiadi system of INAF (Taffoni et al. 2020; Bertocco et al. 2020). The Euclid Consortium acknowledges the European Space Agency and a number of agencies and institutes that have supported the development of Euclid, in particular the Agenzia Spaziale Italiana, the Austrian Forschungsförderungsgesellschaft funded through BMIMI, the Belgian Science Policy, the Canadian Euclid Consortium, the Deutsches Zentrum für Luft- und Raumfahrt, the DTU Space and the Niels Bohr Institute in Denmark, the French Centre National d’Etudes Spatiales, the Fundação para a Ciência e a Tecnologia, the Hungarian Academy of Sciences, the Ministerio de Ciencia, Innovación y Universidades, the National Aeronautics and Space Administration, the National Astronomical Observatory of Japan, the Netherlandse Onderzoekschool Voor Astronomie, the Norwegian Space Agency, the Research Council of Finland, the Romanian Space Agency, the State Secretariat for Education, Research, and Innovation (SERI) at the Swiss Space Office (SSO), and the United Kingdom Space Agency. A complete and detailed list is available on the Euclid web site (www.euclid-ec.org/consortium/community/).

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Is new version of: Discussion Paper: arXiv:2505.04688 (arXiv)

Ministero dell'Istruzione e del Merito
PRIN 2022 2022NY2ZRS 001
European Union
J53D23019100001
European Space Agency

Submitted: 2025-05-06
Accepted: 2025-11-23
Available: 2026-03-17

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Caltech groups: Division of Physics, Mathematics and Astronomy (PMA) , Infrared Processing and Analysis Center (IPAC)
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Euclid preparation. LXXXIII. The impact of redshift interlopers on the two-point correlation function analysis

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