Euclid preparation: XXI. Intermediate-redshift contaminants in the search for $z>6$ galaxies within the Euclid Deep Survey

S. van Mierlo , K. Caputi (1) , M. Ashby (2) , H. Atek (3) , M. Bolzonella (4) , R. Bowler , G. Brammer , C. Conselice , J. Cuby , P. Dayal , A. Díaz-Sánchez , S. Finkelstein , H. Hoekstra (5) , A. Humphrey (6) , O. Ilbert (7) , H. Mccracken (8) , B. Milvang-Jensen (9) , P. Oesch (10) , R. Pello (11) , G. Rodighiero (12) , M. Schirmer , S. Toft , J. Weaver , S. Wilkins , C. Willott , G. Zamorani , A. Amara , N. Auricchio , M. Baldi , R. Bender , C. Bodendorf , D. Bonino , E. Branchini , M. Brescia , J. Brinchmann , S. Camera , V. Capobianco , C. Carbone , J. Carretero , M. Castellano , S. Cavuoti , A. Cimatti , R. Cledassou (13) , G. Congedo , L. Conversi , Y. Copin (14) , L. Corcione , F. Courbin , A. da Silva , H. Degaudenzi , M. Douspis , F. Dubath , X. Dupac , S. Dusini , S. Farrens , S. Ferriol (14) , M. Frailis , E. Franceschi , P. Franzetti , M. Fumana , S. Galeotta , B. Garilli , W. Gillard (15) , B. Gillis , C. Giocoli , A. Grazian , F. Grupp , S. Haugan , W. Holmes , F. Hormuth , A. Hornstrup , K. Jahnke , M. Kümmel , A. Kiessling , M. Kilbinger , T. Kitching , R. Kohley , M. Kunz , H. Kurki-Suonio , R. Laureijs , S. Ligori , P. Lilje , I. Lloro , E. Maiorano , O. Mansutti , O. Marggraf , K. Markovic , F. Marulli , R. Massey , S. Maurogordato (16) , E. Medinaceli , M. Meneghetti , E. Merlin , G. Meylan , M. Moresco , L. Moscardini , E. Munari , S. Niemi , C. Padilla , S. Paltani , F. Pasian , K. Pedersen , V. Pettorino , S. Pires , M. Poncet , L. Popa , L. Pozzetti , F. Raison , A. Renzi , J. Rhodes , G. Riccio , E. Romelli , E. Rossetti , R. Saglia , D. Sapone , B. Sartoris , P. Schneider , A. Secroun (15) , C. Sirignano , G. Sirri , L. Stanco , J.-L. Starck , C. Surace , P. Tallada-Crespí , A. Taylor , I. Tereno , R. Toledo-Moreo , F. Torradeflot , I. Tutusaus , E. Valentijn , L. Valenziano , T. Vassallo , Y. Wang , A. Zacchei , J. Zoubian (15) , S. Andreon , S. Bardelli , A. Boucaud (17) , J. Graciá-Carpio , D. Maino , N. Mauri , S. Mei (17) , F. Sureau , E. Zucca , H. Aussel , C. Baccigalupi , A. Balaguera-Antolínez , A. Biviano , A. Blanchard , S. Borgani , E. Bozzo , C. Burigana , R. Cabanac , F. Calura , A. Cappi , C. Carvalho , S. Casas , G. Castignani , C. Colodro-Conde , A. Cooray , J. Coupon , H. Courtois (14) , M. Crocce , O. Cucciati , S. Davini , H. Dole , J. Escartin , S. Escoffier (15) , M. Fabricius , M. Farina , K. Ganga (17) , J. García-Bellido , K. George , F. Giacomini , G. Gozaliasl , S. Gwyn , I. Hook , M. Huertas-Company , V. Kansal , A. Kashlinsky , E. Keihanen , C. Kirkpatrick , V. Lindholm , R. Maoli , M. Martinelli , N. Martinet (7) , M. Maturi , R. Metcalf , P. Monaco , G. Morgante , A. Nucita , L. Patrizii , A. Peel , J. Pollack (17) , V. Popa , C. Porciani , D. Potter , P. Reimberg , A. Sánchez , V. Scottez , E. Sefusatti , J. Stadel , R. Teyssier , J. Valiviita , M. Viel

1 University of Groningen [Groningen]
2 DLO Winand Staring Centre
3 IAP - Institut d'Astrophysique de Paris
4 University of Bologna/Università di Bologna
5 Department of Surgical Oncology
6 Korea Astronomy and Space Science Institute (KASI)
7 LAM - Laboratoire d'Astrophysique de Marseille
8 IAP - Institut d'Astrophysique de Paris
9 DARK - Dark Cosmology Centre
10 UNIGE - Université de Genève = University of Geneva
11 UMS 831 - Services communs OMP
12 Unipd - Università degli Studi di Padova = University of Padua
13 IN2P3 - Institut National de Physique Nucléaire et de Physique des Particules du CNRS
14 IP2I Lyon - Institut de Physique des 2 Infinis de Lyon
15 CPPM - Centre de Physique des Particules de Marseille
16 LAGRANGE - Joseph Louis LAGRANGE
17 APC (UMR_7164) - AstroParticule et Cosmologie

Context. The Euclid mission is expected to discover thousands of z > 6 galaxies in three deep fields, which together will cover a ∼50 deg 2 area. However, the limited number of Euclid bands (four) and the low availability of ancillary data could make the identification of z > 6 galaxies challenging. Aims. In this work we assess the degree of contamination by intermediate-redshift galaxies ( z = 1–5.8) expected for z > 6 galaxies within the Euclid Deep Survey. Methods. This study is based on ∼176 000 real galaxies at z = 1–8 in a ∼0.7 deg 2 area selected from the UltraVISTA ultra-deep survey and ∼96 000 mock galaxies with 25.3 ≤ H < 27.0, which altogether cover the range of magnitudes to be probed in the Euclid Deep Survey. We simulate Euclid and ancillary photometry from fiducial 28-band photometry and fit spectral energy distributions to various combinations of these simulated data. Results. We demonstrate that identifying z > 6 galaxies with Euclid data alone will be very effective, with a z > 6 recovery of 91% (88%) for bright (faint) galaxies. For the UltraVISTA-like bright sample, the percentage of z = 1–5.8 contaminants amongst apparent z > 6 galaxies as observed with Euclid alone is 18%, which is reduced to 4% (13%) by including ultra-deep Rubin ( Spitzer ) photometry. Conversely, for the faint mock sample, the contamination fraction with Euclid alone is considerably higher at 39%, and minimised to 7% when including ultra-deep Rubin data. For UltraVISTA-like bright galaxies, we find that Euclid ( I E − Y E ) > 2.8 and ( Y E − J E ) < 1.4 colour criteria can separate contaminants from true z > 6 galaxies, although these are applicable to only 54% of the contaminants as many have unconstrained ( I E − Y E ) colours. In the best scenario, these cuts reduce the contamination fraction to 1% whilst preserving 81% of the fiducial z > 6 sample. For the faint mock sample, colour cuts are infeasible; we find instead that a 5 σ detection threshold requirement in at least one of the Euclid near-infrared bands reduces the contamination fraction to 25%.

Domains: Sciences of the Universe [physics]; Sciences of the Universe [physics]; Astrophysics [astro-ph]

S. van Mierlo, K. Caputi, M. Ashby, H. Atek, M. Bolzonella, et al.. Euclid preparation: XXI. Intermediate-redshift contaminants in the search for $z>6$ galaxies within the Euclid Deep Survey. Astronomy and Astrophysics - A&A, 2022, 666, pp.A200. ⟨10.1051/0004-6361/202243950⟩. ⟨hal-03866172⟩