A QSO host galaxy and its Lyalpha emission at z=6.43
  We report an optical detection of an extended structure around a QSO at
z=6.43 (CFHQSJ2329-0301, the highest redshift QSO currently known) in deep z'
and z_r-band images of the Subaru/Suprime-Cam. After a careful PSF (QSO)
subtraction, a structure in the z'-band extends more than 4'' on the sky
(R_e=11 kpc), and thus, is well-resolved (16sigma detection). The
PSF-subtracted z_r-band structure is in a similar shape to that in the z'-band,
but less significant with a 3 sigma detection. In the z'-band, a radial profile
of the QSO+host shows a clear excess over that of the averaged PSF in 0.8-3''
radius.
  Since the z'-band includes a Lya emission at z=6.43, the z' flux is perhaps a
mixture of the host (continuum light) and its Lya emission, whereas the
z_r-band flux is from the host. Through a SED modeling, we estimate 40% of the
PSF-subtracted z'-band light is from the host (continuum) and 60% is from Lya
emission. The absolute magnitude of the host is M_{1450}=-23.9 (c.f.
M_{1450}=-26.4 for the QSO). A lower limit of the SFR(Lya) is 1.6 Msun yr^{-1}
with stellar mass ranging 6.2 x 10^8 to 1.1 x 10^10 Msun when 100 Myrs of age
is assumed. The detection shows that a luminous QSO is already harbored by a
large, star-forming galaxy in the early Universe only after ~840 Myr after the
big bang. The host may be a forming giant galaxy, co-evolving with a super
massive black hole.
