A complete search for redshift z>6.5 quasars in the VIKING survey
  We present the results of a new, deeper, and complete search for
high-redshift $6.5<z<9.3$ quasars over 977deg$^2$ of the VISTA Kilo-Degree
Infrared Galaxy (VIKING) survey. This exploits a new list-driven dataset
providing photometry in all bands ZYJHKs, for all sources detected by VIKING in
$J$. We use the Bayesian model comparison (BMC) selection method of Mortlock et
al., producing a ranked list of just 21 candidates. The sources ranked 1, 2, 3
and 5 are the four known $z>6.5$ quasars in this field. Additional observations
of the other 17 candidates, primarily DESI Legacy Survey photometry and ESO
FORS2 spectroscopy, confirm that none is a quasar. This is the first complete
sample from the VIKING survey, and we provide the computed selection function.
We include a detailed comparison of the BMC method against two other selection
methods: colour cuts and minimum-$\chi^2$ SED fitting. We find that: i) BMC
produces eight times fewer false positives than colour cuts, while also
reaching 0.3 mag. deeper, ii) the minimum-$\chi^2$ SED fitting method is
extremely efficient but reaches 0.7 mag. less deep than the BMC method, and
selects only one of the four known quasars. We show that BMC candidates,
rejected because their photometric SEDs have high $\chi^2$ values, include
bright examples of galaxies with very strong [OIII]$\lambda\lambda$4959,5007
emission in the $Y$ band, identified in fainter surveys by Matsuoka et al. This
is a potential contaminant population in Euclid searches for faint $z>7$
quasars, not previously accounted for, and that requires better
characterisation.
