Giant low surface brightness haloes in distant radio galaxies: USS0828+193

We present results on the spectroscopic study of the ionized gas in the high-redshift radio galaxy USS0828+193 at z = 2.57. Thanks to the high signal-to-noise (S/N) ratio of the emission lines in the Keck spectrum, we have been able to perform a detailed kinematic study by means of the spectral decomposition of the emission line profiles. This study reveals the existence of two types of material in this object: (i) a low surface brightness component with apparent quiescent kinematics consistent with gravitational motions and (ii) a perturbed component with rather extreme kinematics. The quiescent halo extends across the entire object for ∼80 kpc. It is enriched with heavy elements and apparently ionized by the continuum from the active nucleus. The properties of the quiescent halo and its origin are discussed in this paper. We propose that it could be part of a structure that surrounds the entire object, although its nature is not clear (a rotating disc? low surface brightness satellites? a cooling flow nebula? material ejected in galactic winds? other?).