Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs). IX.
  Identification of Two Red Quasars at z > 5.6
  We present the first discovery of dust-reddened quasars (red quasars) in the
high-z universe (z >5.6). This is a result from the Subaru High-z Exploration
of Low-Luminosity Quasars (SHELLQs) project, which is based on the sensitive
multi-band optical imaging data produced by the Hyper Suprime-Cam (HSC) Subaru
Strategic Program survey. We identified four red quasar candidates from the
spectroscopically confirmed 93 high-z quasars in the SHELLQs sample, based on
detections in the Wide-field Infrared Survey Explorer (WISE) data at 3.4 and
4.6 um (rest-frame ~5000-6500 A). The amount of dust reddening was estimated
with spectral energy distribution (SED) fits over optical and mid-infrared
wavelengths. Two of the four candidates were found to be red quasars with dust
reddening of E(B-V) > 0.1. The remaining SHELLQs quasars without individual
WISE detections are significantly fainter in the WISE bands and bluer than the
red quasars, although we did detect them in the W1 band in a stacked image. We
also conducted the same SED fits for high-z optically-luminous quasars, but no
red quasar was found. This demonstrates the power of Subaru HSC to discover
high-z red quasars, which are fainter than the limiting magnitudes of past
surveys in the rest-frame ultraviolet, due to dust extinction.
