Discovering bright quasars at intermediate redshifts based on the
  optical/near-IR colors
  Identifications of quasars at intermediate redshifts (2.2<z<3.5) are
inefficient in most previous quasar surveys as their optical colors are similar
to those of stars. The near-IR K-band excess technique has been suggested to
overcome this difficulty. Our study also proposed to use optical/near-IR colors
for selecting z<4 quasars. To this method, we selected 105 unidentified bright
targets with i<18.5 from the quasar candidates of SDSS DR6 with both SDSS ugriz
optical and UKIDSS YJHK near-IR photometric data, which satisfy our proposed
Y-K/g-z criterion and have photometric redshifts between 2.2 and 3.5 estimated
from the 9-band SDSS-UKIDSS data. 43 of them were observed with the 2.16m
telescope of NAOC in 2012. 36 of them were identified as quasars at 2.1<z<3.4.
High success rate of discovering these quasars in the SDSS spectroscopic
surveyed area demonstrates the robustness of both the Y-K/g-z selection
criterion and the photometric redshift estimation technique. We also used the
above criterion to investigate the possible star contamination rate to the
quasar candidates of SDSS DR6, and found that it is much higher in selecting
3<z<3.5 quasar candidates than selecting lower redshift ones (z<2.2). The
significant improvement in the photometric redshift estimation by using the
9-band SDSS-UKIDSS data than using the 5-band SDSS data is demonstrated and a
catalog of 7,727 unidentified quasar candidates with photometric redshifts
between 2.2 and 3.5 is provided. We also tested the Y-K/g-z selection criterion
with the SDSS-III/DR9 quasar catalog, and found 96.2% of 17,999 DR9 quasars
with UKIDSS Y and K-band data satisfy our criterion. With some samples of red
and type II quasars, we found that 88% and 96.5% of them can be selected by the
Y-K/g-z criterion respectively, which supports that using the Y-K/g-z criterion
we can efficiently select both unobscured and obscured quasars. (abridged)
