A GIANT LY-ALPHA CLOUD ASSOCIATED WITH A DUST-LANE RADIO GALAXY AT Z=2.468

We report the discovery of a giant cloud of Ly alpha emission (z = 2.4 68) associated with an ultra-steep spectrum radio galaxy. The cloud is seen to be confined between the nucleus and the southern hot spot of the double radio source. Its morphology provides evidence for a disk o f dusty material around the parent optical galaxy, which probably obsc ures any Ly alpha emission that may be associated with the far side of the radio galaxy. The proposal that the observed cloud is spatially c oincident with the radio lobe approaching us is substantiated by the m easured velocity gradient across the cloud. Further, an analogous situ ation is shown to exist in some other high-z radio galaxies where the Ly alpha emission exhibits similarly a large asymmetry about the radio nucleus. Circum-galactic dusty disks may, thus, be a fairly common fe ature of high-z radio galaxies. The (tilted) Ly alpha line of the pres ent object has an extremely large equivalent width (similar to 1000 An gstrom in the rest frame) and is found to be clearly split into two ve locity components. The kinematical interpretation of this result is th at the nebula is expanding at a velocity of approximate to 550 kms(-1) driven by the radio lobe it surrounds. Our data thus provides a kinem atical estimate for the age of the radio source of approximate to 2 x 10(7) yrs.