  The faint-end slope of the quasar luminosity function at z~6 and its
implication on the role of quasars in reionizing the intergalactic medium at
early times has been an outstanding problem for some time. The identification
of faint high-redshift quasars with luminosities of <1e44.5 erg/s is
challenging. They are rare (few per square degree) and the separation of these
unresolved quasars from late-type stars and compact star-forming galaxies is
difficult from ground-based observations alone. In addition, source confusion
becomes significant at >25mag, with ~30% of sources having their flux
contaminated by foreground objects when the seeing resolution is ~0.7". We
mitigate these issues by performing a pixel-level joint processing of ground
and space-based data from Subaru/HSC and HST/ACS. We create a deconfused
catalog over the 1.64 square-degrees of the COSMOS field, after accounting for
spatial varying PSFs and astrometric differences between the two datasets. We
identify twelve low-luminosity (M_UV ~ -21 mag) z>6 quasar candidates through
(i) their red color measured between ACS/F814W and HSC/i-band and (ii) their
compactness in the space-based data. We estimate that late-type stars could
contribute up to 50% to our sample. Our constraints on the faint end of the
quasar luminosity function at z~6.4 suggests a negligibly small contribution to
reionization compared to the star-forming galaxy population. The confirmation
of our candidates and the evolution of number density with redshift could
provide better insights into how supermassive galaxies grew in the first
billion years of cosmic time.
