The extended Lyman-α emission surrounding the z=3.04 radio-quiet QSO1205-30:: Primordial infalling gas illuminated by the quasar?

We present spectroscopic observations obtained with the FORS1 instrument on the ESO VLT under good seeing conditions of the radio-quiet z(em) = 3.04 quasar Q1205-30 and its associated extended Ly alpha emission. The extended Ly alpha emission was originally found in a deep narrow band image targeting a z(abs) approximate to z(em) Lyman-limit system in the spectrum of the QSO. Using spectral point-spread function fitting to subtract the QSO spectrum, we clearly detect the extended Ly alpha emission as well as two foreground galaxies at small impact parameters ( 2.12 +/- 0.04 and 2.77 +/- 0.07 arcsec). The redshifts of the two foreground galaxies are found to be z = 0.4732 and z = 0.865. We determine the redshift and velocity profile for the extended Ly alpha emission, and analyzing the velocity offsets between eight QSO emission lines we refine the quasar redshift determination. We use the new redshifts to infer the geometry of the complex. We find that the extended Ly alpha emission is clearly associated with the quasar. A Ly alpha luminosity of 5.6 x 10(43) erg s(-1) places this extended emission at the high luminosity end of the few previous detections around radio-quiet quasars. The extended Ly alpha emission is best explained by hydrogen falling into the dark matter halo inhabited by the quasar.