We report the discovery of a large cloud of ionized gas associated with the radio galaxy 3C 294 at a redshift of 1.786. The radio source is a powerful double with a weak core. We detect Lyα emission with a total monochromatic luminosity of L_(Lyα) = 7.6 x 10^(44) ergs s^(-1) extended over ~100 x 170 kpc. The emission-line cloud is highly elongated and is well aligned with the inner radio source axis. Long-slit spectra, taken along the major axis, show spatially extended emission lines of N v λ1240, C IV λ1550, He II λ1640, and C III λ1909. The extended Lyα emission shows a large, smooth velocity gradient (1500 km s^(-1) and large intrinsic line widths (700-2600 km s^(-1)). The high-ionization lines, particularly C IV, show large velocity changes that are systematically different from those of Lyα.
3C 294 appears to be another, even more spectacular member of a class of powerful radio galaxies at high redshift typified by 3C 326.1. We discuss our observations from the point of view of star formation and the interaction of nuclear activity with ambient material. We consider two scenarios for the ionization of the Lyα cloud: ionization by a population of extremely massive stars and ionization by a non thermal continuum from the radio nucleus. Stellar photoionization appears to be insufficient in both the total number of ionizing photons and its ability to produce highly ionized species. We propose that the 150 kpc cloud of gas is photoionized by a central nonstellar source. We detect a very red compact object at 2.2 μm nearly coincident with the radio core. It is unclear whether the light from this object is primarily stellar or nonstellar
