The Infrared Medium-Deep Survey. V. A New Selection Strategy for Quasars
  at z > 5 based on Medium-Band Observation with SQUEAN
  Multiple color selection techniques have been successful in identifying
quasars from wide-field broad-band imaging survey data. Among the quasars that
have been discovered so far, however, there is a redshift gap at $5 \lesssim
{\rm z} \lesssim 5.7$ due to the limitations of filter sets in previous
studies. In this work, we present a new selection technique of high redshift
quasars using a sequence of medium-band filters: nine filters with central
wavelengths from 625 to 1025 nm and bandwidths of 50 nm. Photometry with these
medium-bands traces the spectral energy distribution (SED) of a source, similar
to spectroscopy with resolution R $\sim$ 15. By conducting medium-band
observations of high redshift quasars at 4.7 $\leq$ z $\leq$ 6.0 and brown
dwarfs (the main contaminants in high redshift quasar selection) using the SED
camera for QUasars in EArly uNiverse (SQUEAN) on the 2.1-m telescope at the
McDonald Observatory, we show that these medium-band filters are superior to
multi-color broad-band color section in separating high redshift quasars from
brown dwarfs. In addition, we show that redshifts of high redshift quasars can
be determined to an accuracy of $\Delta{\rm z}/(1+{\rm z}) = 0.002$ -- $0.026$.
The selection technique can be extended to z $\sim$ 7, suggesting that the
medium-band observation can be powerful in identifying quasars even at the
re-ionization epoch.
