Spectroscopy of the spatially extended Lyα emission around a quasar at z= 6.4


E-mail: tomo@ifa.hawaii.edu


We have taken deep, moderate-resolution Keck/DEIMOS spectra of the quasar (QSO) CFHQS J232908.28−030158.8 at z= 6.4. At the wavelength of Lyα, the spectrum shows a spatially extended component, which is significantly more extended than a stellar spectrum, and also a continuum part of the spectrum. The rest-frame linewidth of the extended component is 21 ± 7 Å  and thus smaller than that of the QSO (52 ± 4 Å), where they should be identical if the light is incomplete subtraction of the QSO component. Therefore, these comparisons argue for the detection of spatially extended Lyα nebulae around this QSO. This is the first z > 6 QSO where an extended Lyα halo has been observed. Careful subtraction of the central QSO spectrum reveals a lower limit to the Lyα luminosity of (1.7 ± 0.1) × 1043 erg s−1. This emission may be from the theoretically predicted infalling gas in the process of forming a primordial galaxy that is ionized by a central QSO. On the other hand, if it is photoionized by the host galaxy, an estimated star formation rate of >3.0 M yr−1 is required.

If we assume the gas is virialized, we obtain a dynamical mass estimate of Mdyn= 1.2 × 1012 M. The derived MBH/Mhost ratio is 2.1 × 10−4, which is two orders of magnitude smaller than those from more massive z∼ 6 QSOs, and places this galaxy in accordance with the localMBH–σ relation, in contrast to a previous claim on the evolution of the MBH–σ relation at z∼ 6. We do not claim evolution or non-evolution of the MBH–σ relation based on a single object, but our result highlights the importance of investigating fainter QSOs at z∼ 6.