A QSO host galaxy and its Lyα emission at z=6.43☆

Host galaxies of highest redshift quasi-stellar objects (QSOs) are of interest; they provide us with a valuable opportunity to investigate physics relevant to the starburst-active galactic nuclei (AGN) connection at the earliest epoch of the Universe, with the most luminous black holes. Here, we report an optical detection of an extended structure around a QSO at z = 6.43 in deep z'- and z(r)-band images of the Subaru/Suprime-Cam. Our target is CFHQS J2329-0301 (z = 6.43), the highest redshift QSO currently known. We have carefully subtracted a point spread function (PSF) constructed using nearby stars from the images. After the PSF (QSO) subtraction, a structure in the z' band extends more than 4 arcsec on the sky (R-e = 11 kpc), and, thus, is well resolved (16 Sigma 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 arcsec radius. Since the z' band includes a Ly alpha emission at z = 6.43, we suggest the z' flux is a mixture of the host (continuum light) and its Ly alpha emission, whereas the z(r)-band flux is from the host. Through a SED modelling, we estimate 40 per cent of the PSF-subtracted z'-band light is from the host (continuum) and 60 per cent is from Ly alpha emission. The absolute magnitude of the host is M-1450 = -23.9 (cf. M-1450 = -26.4 for the QSO). A lower limit of the SFR(Ly alpha) is 1.6 M-circle dot yr-1 with stellar mass ranging from 6.2 x 108 to 1.1 x 1010 M-circle dot when 100 Myr of age is assumed. The detection shows that a luminous QSO is already harboured by a large, star-forming galaxy in the early Universe only after similar to 840 Myr after the big bang. The host may be a forming giant galaxy, co-evolving with a super-massive black hole.