Euclidpreparation
Authors:   Euclid Collaboration, Barnett R. , Warren S. J. , Mortlock D. J. , Cuby J.-G. , Conselice C. , Hewett P. C. , Willott C. J. , Auricchio N. , Balaguera-Antolínez A. , Baldi M. , Bardelli S. , Bellagamba F. , Bender R. , Biviano A. , Bonino D. , Bozzo E. , Branchini E. , Brescia M. , Brinchmann J. , Burigana C. , Camera S. , Capobianco V. , Carbone C. , Carretero J. , Carvalho C. S. , Castander F. J. , Castellano M. , Cavuoti S. , Cimatti A. , Clédassou R. , Congedo G. , Conversi L. , Copin Y. , Corcione L. , Coupon J. , Courtois H. M. , Cropper M. , Da Silva A. , Duncan C. A. J. , Dusini S. , Ealet A. , Farrens S. , Fosalba P. , Fotopoulou S. , Fourmanoit N. , Frailis M. , Fumana M. , Galeotta S. , Garilli B. , Gillard W. , Gillis B. R. , Graciá-Carpio J. , Grupp F. , Hoekstra H. , Hormuth F. , Israel H. , Jahnke K. , Kermiche S. , Kilbinger M. , Kirkpatrick C. C. , Kitching T. , Kohley R. , Kubik B. , Kunz M. , Kurki-Suonio H. , Laureijs R. , Ligori S. , Lilje P. B. , Lloro I. , Maiorano E. , Mansutti O. , Marggraf O. , Martinet N. , Marulli F. , Massey R. , Mauri N. , Medinaceli E. , Mei S. , Mellier Y. , Metcalf R. B. , Metge J. J. , Meylan G. , Moresco M. , Moscardini L. , Munari E. , Neissner C. , Niemi S. M. , Nutma T. , Padilla C. , Paltani S. , Pasian F. , Paykari P. , Percival W. J. , Pettorino V. , Polenta G. , Poncet M. , Pozzetti L. , Raison F. , Renzi A. , Rhodes J. , Rix H.-W. , Romelli E. , Roncarelli M. , Rossetti E. , Saglia R. , Sapone D. , Scaramella R. , Schneider P. , Scottez V. , Secroun A. , Serrano S. , Sirri G. , Stanco L. , Sureau F. , Tallada-Crespí P. , Tavagnacco D. , Taylor A. N. , Tenti M. , Tereno I. , Toledo-Moreo R. , Torradeflot F. , Valenziano L. , Vassallo T. , Wang Y. , Zacchei A. , Zamorani G. , Zoubian J. , Zucca E. 
Published: September 2019

We provide predictions of the yield of 7 <  z <  9 quasars from theEuclidwide survey, updating the calculation presented in theEuclidRed Book in several ways. We account for revisions to theEuclidnear-infrared filter wavelengths; we adopt steeper rates of decline of the quasar luminosity function (QLF; Φ) with redshift, Φ ∝ 10k(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 toJAB ∼ 23. Quasars atz >  8 may be selected fromEuclidOYJHphotometry alone, but selection over the redshift interval 7 <  z <  8 is greatly improved by the addition ofz-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 beyondz = 6. If the decline of the QLF accelerates beyondz = 6, withk = −0.92,Euclidshould nevertheless find over 100 quasars with 7.0 <  z <  7.5, and ∼25 quasars beyond the current record ofz = 7.5, including ∼8 beyondz = 8.0. The firstEuclidquasars atz >  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,M1450 <  −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 atJAB ∼ 23. The precision with whichkcan be determined over 7 <  z <  8 depends on the value ofk, but assumingk = −0.72 it can be measured to a 1σuncertainty of 0.07.