A giant Lyα cloud associated with a dust-lane radio galaxy at Z = 2.468

Abstract

We report the discovery of a giant cloud of Lyα emission (z = 2.468) 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 of dusty material around the parent optical galaxy, which probably obscures any Lyα emission that may be associated with the far side of the radio galaxy. The proposal that the observed cloud is spatially coincident with the radio lobe approaching us is substantiated by the measured velocity gradient across the cloud. Further, an analogous situation is shown to exist in some other high-z radio galaxies where the Lyα emission exhibits similarly a large asymmetry about the radio nucleus. Circum-galactic dusty disks may, thus, be a fairly common feature of high-z radio galaxies. The (tilted) Lyα line of the present object has an extremely large equivalent width (~1000 Å in the rest frame) and is found to be clearly split into two velocity components. The kinematical interpretation of this result is that the nebula is expanding at a velocity of ≈ 550kms^-1 driven by the radio lobe it surrounds. Our data thus provides a kinematical estimate for the age of the radio source of ≈ 2 × 10⁷ yrs.