A giant low-ionization nebula associated with a radio galaxy in a forming cluster at z=0.79

We report an exceptionally interesting case of a powerful ultra-steep spectrum radio galaxy at z = 0.794 which exhibits an extended emission line region (EELR) of size about 100 kpc. The galaxy, with an I-band magnitude of 20.1, is apparently located in a forming cluster of galaxies, which may itself be part of a larger structure. The cluster identification is supported by the spectroscopic redshifts measured for another two galaxies which are located in the vicinity of the radio galaxy. The outer radio contours of the S-shaped double radio source show a striking spatial coincidence with the edges of the EELR, suggesting that the latter has formed from cooling of the intracluster medium (ICM) compressed by the radio lobes. We also consider the possibility that the EELR has originated in tidal interactions/mergers among gas-rich galaxies near the cluster core, as hinted by a long filament of stellar material seen to link the radio-loud galaxy with a neighbour object. The EELR is marked by spectacular sharp-edged structures resembling cones of ionizing radiation emanating from the nucleus of the radio galaxy. From the observed large ratio of [OII]/[OIII] similar to 3, the excitation level of the EELR is inferred to be very low, consistent with other cluster cooling flow nebula. The host of the radio source will possibly, once the merging process and the radio activity are over, become the central cD galaxy of the cluster.