A z=3 Ly alpha BLOB ASSOCIATED WITH A DAMPED Ly alpha SYSTEM PROXIMATE TO ITS BACKGROUND QUASAR

We report on the discovery of a bright Ly alpha blob associated with the z = 3 quasar SDSS J124020.91+145535.6 which is also coincident with strong damped Ly alpha absorption from a foreground galaxy (a so-called proximate damped Ly alpha (PDLA) system). The one-dimensional spectrum acquired by the Sloan Digital Sky Survey (SDSS) shows a broad Ly alpha emission line with a FWHM similar or equal to 500 km s(-1) and a luminosity of L-Ly alpha = 3.9 x 10(43) erg s(-1) superposed on the trough of the PDLA. Follow-up observations using the Keck/LRIS spectrometer confirm that this source has a Ly alpha nebula with spatial extent exceeding 5 '', corresponding to a proper size > 39 kpc. Mechanisms for powering the large Ly alpha luminosity in this nebula are discussed. We use a Monte Carlo radiative transfer simulation to investigate the possibility that the line emission is fluorescent recombination radiation from a kpc-scale PDLA galaxy powered by the ionizing flux of the quasar, but find that the predicted Ly alpha flux is several orders of magnitude lower than observed. We conclude that the Ly alpha emission is not associated with the PDLA galaxy at all, but instead is intrinsic to the quasar's host and similar to the extended Ly alpha "fuzz" which is detected around many active galactic nuclei. PDLAs are natural coronagraphs that block their background quasar at Ly alpha and we discuss how systems similar to SDSS J124020.91+145535.6 might be used to image the neutral hydrogen in the PDLA galaxy in silhouette against the screen of extended Ly alpha emission from the background quasar.