SDSS quasars in the WISE preliminary data release and quasar candidate
  selection with optical/infrared colors
  We present a catalog of 37,842 quasars in the SDSS Data Release 7, which have
counterparts within 6" in the WISE Preliminary Data Release. The overall WISE
detection rate of the SDSS quasars is 86.7%, and it decreases to less than
50.0% when the quasar magnitude is fainter than $i=20.5$. We derive the median
color-redshift relations based on this SDSS-WISE quasar sample and apply them
to estimate the photometric redshifts of the SDSS-WISE quasars. We find that by
adding the WISE W1- and W2-band data to the SDSS photometry we can increase the
photometric redshift reliability, defined as the percentage of sources with the
photometric and spectroscopic redshift difference less than 0.2, from 70.3% to
77.2%. We also obtain the samples of WISE-detected normal and late-type stars
with SDSS spectroscopy, and present a criterion in the $z-W1$ versus $g-z$
color-color diagram, $z-W1>0.66(g-z)+2.01$, to separate quasars from stars.
With this criterion we can recover 98.6% of 3089 radio-detected SDSS-WISE
quasars with redshifts less than four and overcome the difficulty in selecting
quasars with redshifts between 2.2 and 3 from SDSS photometric data alone. We
also suggest another criterion involving the WISE color only, $W1-W2>0.57$, to
efficiently separate quasars with redshifts less than 3.2 from stars. In
addition, we compile a catalog of 5614 SDSS quasars detected by both WISE and
UKIDSS surveys and present their color-redshift relations in the optical and
infrared bands. By using the SDSS $ugriz$, UKIDSS YJHK and WISE W1- and W2-band
photometric data, we can efficiently select quasar candidates and increase the
photometric redshift reliability up to 87.0%. We discuss the implications of
our results on the future quasar surveys. An updated SDSS-WISE quasar catalog
consisting of 101,853 quasars with the recently released WISE all-sky data is
also provided.
