A QSO host galaxy and its Ly�� emission atz= 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 zr-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 (Re= 11 kpc), and, thus, is well resolved (16σ detection). The PSF-subtracted zr-band structure is in a similar shape to that in the z′ band, but less significant with a 3σ 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 Lyα emission at z= 6.43, we suggest the z′ flux is a mixture of the host (continuum light) and its Lyα emission, whereas the zr-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 Lyα emission. The absolute magnitude of the host is M1450=−23.9 (cf. M1450=−26.4 for the QSO). A lower limit of the SFR(Lyα) is 1.6 M⊙ yr−1 with stellar mass ranging from 6.2 × 108 to 1.1 × 1010 M⊙ 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 ∼840 Myr after the big bang. The host may be a forming giant galaxy, co-evolving with a super-massive black hole.