A survey of luminous high-redshift quasars with SDSS and WISE. I. target
  selection and optical spectroscopy
  High-redshift quasars are important tracers of structure and evolution in the
early universe. However, they are very rare and difficult to find when using
color selection because of contamination from late-type dwarfs. High-redshift
quasar surveys based on only optical colors suffer from incompleteness and low
identification efficiency, especially at $z\gtrsim4.5$. We have developed a new
method to select $4.7\lesssim z \lesssim 5.4$ quasars with both high efficiency
and completeness by combining optical and mid-IR Wide-field Infrared Survey
Explorer (WISE) photometric data, and are conducting a luminous $z\sim5$ quasar
survey in the whole Sloan Digital Sky Survey (SDSS) footprint. We have
spectroscopically observed 99 out of 110 candidates with $z$-band magnitudes
brighter than 19.5 and 64 (64.6\%) of them are quasars with redshifts of
$4.4\lesssim z \lesssim 5.5$ and absolute magnitudes of $-29\lesssim M_{1450}
\lesssim -26.4$. In addition, we also observed 14 fainter candidates selected
with the same criteria and identified 8 (57.1\%) of them as quasars with
$4.7<z<5.4$ . Among 72 newly identified quasars, 12 of them are at $5.2 < z <
5.7$, which leads to an increase of $\sim$36\% of the number of known quasars
at this redshift range. More importantly, our identifications doubled the
number of quasars with $M_{1450}<-27.5$ at $z>4.5$, which will set strong
constraints on the bright end of the quasar luminosity function. We also expand
our method to select quasars at $z\gtrsim5.7$. In this paper we report the
discovery of four new luminous $z\gtrsim5.7$ quasars based on SDSS-WISE
selection.
