*Euclid*preparation

V. Predicted yield of redshift 7 < z < 9 quasars from the wide survey

*Euclid* Collaboration, R. Barnett, S. Warren, D. Mortlock, J.-G. Cuby, C. J. Conselice, P. C. Hewett, C. J. Willott, N. Auricchio, A. Balaguera-Antolínez, M. Baldi, S. Bardelli, F. Bellagamba, R. Bender, A. Biviano, D. Bonino, E. Bozzo, E. Branchini, M. Brescia, **J. Brinchmann**, C. Burigana, S. Camera, V. Capobianco, C. Carbone, J. Carretero, **C. S. Carvalho**, F. J. Castander, M. Castellano, S. Cavuoti, A. Cimatti, R. Clédassou, G. Congedo, L. Conversi, Y. Copin, L. Corcione, J. Coupon, H. M. Courtois, M. Cropper, **A. C. da Silva**, C. A. J. Duncan, S. Dusini, A. Ealet, S. Farrens, P. Fosalba, S. Fotopoulou, N. Fourmanoit, M. Frailis, M. Fumana, S. Galeotta, B. Garilli, W. Gillard, B. Gillis, J. Graciá-Carpio, F. Grupp, H. Hoekstra, F. Hormuth, H. Israel, K. Jahnke, S. Kermiche, M. Kilbinger, C. C. Kirkpatrick, T. Kitching, R. Kohley, B. Kubik, M. Kunz, H. Kurki-Suonio, R. J. Laureijs, S. Ligori, P. B. Lilje, I. Lloro, E. Maiorano, O. Mansutti, O. Marggraf, N. Martinet, F. Marulli, R. Massey, N. Mauri, E. Medinaceli, S. Mei, Y. Mellier, R. B. Metcalf, J. J. Metge, G. Meylan, M. Moresco, L. Moscardini, E. Munari, C. Neissner, S. Niemi, T. Nutma, C. Padilla, S. Paltani, F. Pasian, P. Paykari, W. J. Percival, V. Pettorino, G. Polenta, M. Poncet, L. Pozzetti, F. Raison, A. Renzi, J. D. Rhodes, H.-W. Rix, E. Romelli, M. Roncarelli, E. Rossetti, R. Saglia, D. Sapone, R. Scaramella, P. Schneider, V. Scottez, A. Secroun, S. Serrano, G. Sirri, L. Stanco, F. Sureau, P. Tallada-Crespí, D. Tavagnacco, A. N. Taylor, M. Tenti, **I. Tereno**, R. Toledo-Moreo, F. Torradeflot, L. Valenziano, T. Vassallo, Y. Wang, A. Zacchei, G. Zamorani, J. Zoubian, E. Zucca

**Abstract**

We provide predictions of the yield of 7 < * z *< 9 quasars from the Euclid wide survey, updating the calculation presented in the Euclid Red Book in several ways. We account for revisions to the *Euclid* near-infrared filter wavelengths; we adopt steeper rates of decline of the quasar luminosity function (QLF; Φ) with redshift, Φ ∝ 10^{k(z − 6)}, *k *= −0.72, and a further steeper rate of decline, *k *= −0.92; we use better models of the contaminating populations (MLT dwarfs and compact early-type galaxies); and we make use of an improved Bayesian selection method, compared to the colour cuts used for the Red Book calculation, allowing the identification of fainter quasars, down to J_{AB} ∼ 23. Quasars at z > 8 may be selected from *Euclid OYJH* photometry alone, but selection over the redshift interval 7 < * z *< 8 is greatly improved by the addition of z-band data from, e.g., Pan-STARRS and LSST. We calculate predicted quasar yields for the assumed values of the rate of decline of the QLF beyond *z *= 6. If the decline of the QLF accelerates beyond *z *= 6, with *k* = −0.92, Euclid should nevertheless find over 100 quasars with 7.0 < *z *< 7.5, and ∼25 quasars beyond the current record of *z *= 7.5, including ∼8 beyond *z* = 8.0. The first Euclid quasars at *z* > 7.5 should be found in the DR1 data release, expected in 2024. It will be possible to determine the bright-end slope of the QLF, 7 < * z* < 8, M_{1450} < −25, using 8 m class telescopes to confirm candidates, but follow-up with JWST or E-ELT will be required to measure the faint-end slope. Contamination of the candidate lists is predicted to be modest even at J_{AB} ∼ 23. The precision with which k can be determined over 7 < *z *< 8 depends on the value of *k*, but assuming *k* = −0.72 it can be measured to a 1σ uncertainty of 0.07.

**Keywords**

quasars: general; methods: statistical; surveys; Astrophysics - Astrophysics of Galaxies; Astrophysics - Cosmology and Nongalactic Astrophysics

**Astronomy and Astrophysics**

Volume 631, Article Number A85, Number of pages 19

2019 October