The extended Lyman-alpha 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 Lyman-alpha emission. The extended Lyman-alpha emission was originally found in a deep narrow band image targeting a z_abs = 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 Lyman-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 Lyman-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 Lyman-alpha emission is clearly associated with the quasar. A Lyman-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 Lyman-alpha emission is best explained by hydrogen falling into the dark matter halo inhabited by the quasar.