The Infrared Medium-deep Survey. III. Survey of Luminous Quasars at 4.7
  $\leq$ z $\leq$ 5.4
  We present our first results of the survey for high redshift quasars at $5
\lesssim {\rm z} \lesssim 5.7$. The search for quasars in this redshift range
has been known to be challenging due to limitations of filter sets used in
previous studies. We conducted a quasar survey for two specific redshift
ranges, 4.60 $\leq$ z $\leq$ 5.40 and 5.50 $\leq$ z $\leq$ 6.05, using
multi-wavelength data that include observations using custom-designed filters,
$is$ and $iz$. Using these filters and a new selection technique, we were able
to reduce the fraction of interlopers. Through optical spectroscopy, we
confirmed seven quasars at 4.7 $\leq$ z $\leq$ 5.4 with $-27.4 < M_{1450} <
-26.4$ which were discovered independently by another group recently. We
estimated black hole masses and Eddington ratios of four of these quasars from
optical and near-infrared spectra, and found that these quasars are undergoing
nearly Eddington-limited accretion which is consistent with the rapid growth of
supermassive black holes in luminous quasars at z $\sim$ 5.
