HST narrow-band search for extended Ly-alpha emission around two z>6
  quasars
  We search for extended Ly-alpha emission around two z>6 quasars, SDSS
J1030+0524 (z=6.309) and SDSS J1148+5251 (z=6.419) using WFC3 narrow-band
filters on board the Hubble Space Telescope. For each quasar, we collected two
deep, narrow-band images, one sampling the Ly-alpha line+continuum at the
quasar redshifts and one of the continuum emission redwards of the line. After
carefully modeling the Point Spread Function, we find no evidence for extended
Ly-alpha emission. These observations set 2-sigma limits of L(Ly-alpha,
extended) < 3.2 x 10^{44} erg/s for J1030+0524 and L(Ly-alpha, extended) < 2.5
x 10^{44} erg/s for J1148+5251. Given the star formation rates typically
inferred from (rest-frame) far-infrared measurements of z~6 quasars, these
limits are well below the intrinsic bright Ly-alpha emission expected from the
recombination of gas photoionized by the quasars or by the star formation in
the host galaxies, and point towards significant Ly-alpha suppression or dust
attenuation. However, small extinction values have been observed along the line
of sight to the nuclei, thus reddening has to be coupled with other mechanisms
for Ly-alpha suppression (e.g., resonance scattering). No Ly-alpha emitting
companions are found, down to a 5-sigma sensitivity of ~ 1 x 10^{-17}
erg/s/cm^2/arcsec^2 (surface brightness) and ~ 5 x 10^{-17} erg/s/cm^2
(assuming point sources).
