Discovery of A Faint Quasar at z~6 and Implications for Cosmic
  Reionization
  Recent studies suggest that faint active galactic nuclei may be responsible
for the reionization of the universe. Confirmation of this scenario requires
spectroscopic identification of faint quasars ($M_{1450}>-24$ mag) at $z
\gtrsim6$, but only a very small number of such quasars have been
spectroscopically identified so far. Here, we report the discovery of a faint
quasar IMS J220417.92+011144.8 at z~6 in a 12.5 deg$^{2}$ region of the SA22
field of the Infrared Medium-deep Survey (IMS). The spectrum of the quasar
shows a sharp break at $\sim8443~\rm{\AA}$, with emission lines redshifted to
$z=5.944 \pm 0.002$ and rest-frame ultraviolet continuum magnitude
$M_{1450}=-23.59\pm0.10$ AB mag. The discovery of IMS J220417.92+011144.8 is
consistent with the expected number of quasars at z~6 estimated from quasar
luminosity functions based on previous observations of spectroscopically
identified low-luminosity quasars . This suggests that the number of
$M_{1450}\sim-23$ mag quasars at z~6 may not be high enough to fully account
for the reionization of the universe. In addition, our study demonstrates that
faint quasars in the early universe can be identified effectively with a
moderately wide and deep near-infrared survey such as the IMS.
