Observations of radio galaxy MRC 1138-262: merging galaxies embedded in a giant Lyman alpha halo

The radio galaxy MRC 1138-262 at z=2.16 is most likely a brightest cluster galaxy in an early stage of evolution. Here we present observations of the luminous emission line halo and the stellar components of this radio galaxy. Optical narrow band imaging shows a very extended (about 160 kpc) and luminous Lyman alpha halo. Infrared narrow band imaging reveals a much smaller H alpha halo with a very different morphology. We advocate a model in which the inner part of the halo is photoionized by direct AGN illumination or by UV photons from young stars. Far from the nucleus (about 25 kpc), there is a region of greatly enhanced Lyman alpha emission. At this location, it is likely that shock ionization is important as indicated by a bend in the radio jet. Spectroscopy of several continuum clumps in the halo shows that, although there are striking differences between the emission and absorption features of the spectra of various regions, they have properties similar to those of Lyman-break galaxies. This is further evidence for a scenario in which massive galaxies form hierarchically from smaller building blocks.