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

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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 (Laureijs et
al. 2011) 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; $\Phi$) with redshift, $\Phi\propto10^k(z-6)$, $k=-0.72$,
consistent with Jiang et al. (2016), 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 use an improved Bayesian selection
method, compared to the colour cuts used for the Red Book calculation, allowing
the identification of fainter quasars, down to $J_AB\sim23$. 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 $z=6$. For the case
that 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 $\sim25$
quasars beyond the current record of $z=7.5$, including $\sim8$ 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 8m 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
$J_AB\sim23$. 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 1
sigma uncertainty of 0.07.

- URL
- http://arxiv.org/abs/1908.04310
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