The giant star forming halo associated with the radio galaxy PKS 1932-46

We report the discovery of a giant (~ 160 kpc) knotty extended emission line nebula associated with the radio galaxy PKS 1932-46 at z = 0.23. The 2-d long slit spectra, obtained with VLT-FORS2 at a large angle (~63°) to the radio source axis, shows that the nebula extends well beyond the radio structure and the ionization cones of the active nucleus. This is one of the largest ionized nebulae yet detected around a radio galaxy at any redshift. The analysis of the ionization, morphological and kinematic properties of the knots suggests that these are star-forming objects, probably compact HII galaxies. We propose that the giant structure is a star forming halo associated with the debris of the merger that triggered the activity. This study reinforces the view that radio galaxies are activated by major mergers which also trigger substantial star formation. The star formation activity can extend on the scale of a galaxy group, beyond the old stellar halo of the host galaxy.