A POWERFUL RADIO GALAXY AT Z=3.6 IN A GIANT ROTATING LYMAN-ALPHA HALO

We present the discovery and detailed observations of the radio galaxy 1243+036 at a redshift of z = 3.57. The radio source was selected on the basis of its extremely steep radio spectrum, suggesting that it mi ght be very distant. The radio source was identified with a galaxy of R magnitude 22.5. Subsequent spectroscopy showed strong Lyman alpha an d [O III]lambda lambda 5007,4959 emission, indicating that the object is a radio galaxy at z = 3.57. High resolution (0.2'') radio maps show an FRII type radio source with a sharply bent radio structure. Strong depolarization of the radio emission indicates that the source is emb edded in a magneto-ionic medium. The most spectacular feature of 1243036 is the presence of a Ly alpha halo of luminosity similar to 10(44. 5) erg s(-1) which extends over similar to 20'' (135 kPc), A 0.6'' res olution Ly alpha image shows that the emission line gas is aligned wit h the main axis of the radio source and has structure down to the scal e of the resolution. High resolution spectra show that the Ly alpha em itting gas has complex kinematic structure. The gas contained within t he radio structure has a relatively high velocity width (similar to 15 00 km s(-1) FWHM). The component of the Ly alpha emission that coincid es with the bend in the radio structure is blueshifted with respect to the peak of the emission by 1100 km s(-1). There is low surface brigh tness Ly alpha emission aligned with, but extending 40 kpc beyond both sides of the radio source, This halo has a narrow velocity width (sim ilar to 250 km s(-1) FWHM) and a velocity gradient of 450 km s(-1) ove r the extent of the emission. The presence of the quiescent Ly alpha c omponent aligned with the AGN axis, but outside the radio source, is s trong evidence that photoionization by anisotropically emitted radiati on from the active nucleus is occurring. Various mechanisms for the or igin and kinematics of the Ly alpha halo are discussed, Because the ha lo extends beyond the radio structure with less violent and more order ed kinematics than inside the radio structure, we conclude that the ou ter halo and its kinematics must predate the radio source, The ordered motion may be large-scale rotation caused by the accretion of gas fro m the environment of the radio galaxy or by a merger. Although alterna tively the halo may be caused by a massive outflow, we argue that bulk inflow of the emission line gas is inconsistent with the most likely orientation of the radio source. The large velocity-width of the Ly al pha gas contained within the radio source compared to that of the oute r halo suggest a direct interaction of the radio source with the gas. The spatial correlation of enhanced, blue-shifted Ly alpha emission an d the sharp bend of the radio structure suggest that the emission line gas could have deflected the radio jet. The impact of the jet could h ave accelerated the gas at this position and may have locally enhanced the Ly alpha emission. Extended faint optical continuum emission is a ligned with the principal radio axis, a phenomenon commonly observed i n high redshift radio galaxies, This emission does not follow the bend ing of the radio jet, indicating that, at least in 1243+036, models in voking scattering of continuum radiation from the AGN as the cause of this alignment are favoured.