A Lyα HALO AROUND A QUASAR AT REDSHIFT z = 6.4

We present long-slit spectroscopic data that reveal extended Lyα emission around the z = 6.417 radio-quiet quasar CFHQS J2329–0301. The Lyα emission is extended over 15 kpc and has a luminosity of >8 × 1036 W, comparable to the most luminous Lyα halos known. The emission has complex kinematics, in part due to foreground absorption, which only partly covers the extended nebula. The velocity ranges from –500 km s–1 to +500 km s–1, with a peak remarkably close to the systemic velocity identified by broad Mg II emission of the quasar. There is no evidence for infall or outflow of the halo gas. We speculate that the Lyα emission mechanism is recombination after quasar photoionization of gas sitting within a high-mass dark matter halo. The immense Lyα luminosity indicates a higher covering factor of cold gas compared with typical radio-quiet quasars at lower redshift.