1996A&A...313...25V

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 might be very distant. The radio source was identified with a galaxy of R magnitude 22.5. Subsequent spectroscopy showed strong Lyman α and [O III]λλ5007,4959 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 embedded in a magneto-ionic medium. The most spectacular feature of 1243+036 is the presence of a Lyα halo of luminosity ∼10^44.5erg/s which extends over ∼20" (135kpc). A 0.6" resolution Lyα image shows that the emission line gas is aligned with the main axis of the radio source and has structure down to the scale of the resolution. High resolution spectra show that the Lyα emitting gas has complex kinematic structure. The gas contained within the radio structure has a relatively high velocity width (∼1500km/s FWHM). The component of the Lyα emission that coincides with the bend in the radio structure is blueshifted with respect to the peak of the emission by 1100km/s. There is low surface brightness Lyα emission aligned with, but extending 40kpc beyond both sides of the radio source. This halo has a narrow velocity width (∼250km/s FWHM) and a velocity gradient of 450km/s over the extent of the emission. The presence of the quiescent Lyα component aligned with the AGN axis, but outside the radio source, is strong evidence that photoionization by anisotropically emitted radiation from the active nucleus is occurring. Various mechanisms for the origin and kinematics of the Lyα halo are discussed. Because the halo extends beyond the radio structure with less violent and more ordered kinematics than inside the radio structure, we conclude that the outer halo and its kinematics must predate the radio source. The ordered motion may be large-scale rotation caused by the accretion of gas from the environment of the radio galaxy or by a merger. Although alternatively 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 Lyα gas contained within the radio source compared to that of the outer halo suggest a direct interaction of the radio source with the gas. The spatial correlation of enhanced, blue-shifted Lyα emission and 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 have accelerated the gas at this position and may have locally enhanced the Lyα emission. Extended faint optical continuum emission is aligned with the principal radio axis, a phenomenon commonly observed in high redshift radio galaxies. This emission does not follow the bending of the radio jet, indicating that, at least in 1243+036, models invoking scattering of continuum radiation from the AGN as the cause of this alignment are favoured.