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 system ; PDLA ) .
The one dimensional spectrum acquired by the Sloan Digital Sky Survey ( SDSS ) shows a broad Ly \alpha emission line with a FWHM \simeq 500 { km s ^ { -1 } } and a luminosity of L _ { Ly \alpha } = 3.9 { \times 10 ^ { 43 } } { erg s ^ { -1 } } superposed on the trough of the PDLA .
Mechanisms for powering this large Ly \alpha luminosity are discussed .
We argue against emission from H ii regions in the PDLA galaxy since this requires an excessive star-formation rate \sim 500 M _ { \odot } yr ^ { -1 } and would correspond to the largest Ly \alpha luminosity ever measured from a damped Ly \alpha system or starburst galaxy .
We use a Monte Carlo radiative transfer simulation to investigate the possibility that the line emission is fluorescent recombination radiation from the 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 AGN .
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 .