THE RADIO GALAXY 1138-262 AT Z=2.2 - A GIANT ELLIPTIC GALAXY AT THE CENTER OF A PROTO-CLUSTER

We present a detailed observational study of a remarkable radio galaxy at a redshift of z = 2.2:1138-262. This object was selected from our compendium of ultra sleep spectrum radio sources on the basis of its d istorted radio morphology. High resolution VLA radio observations show that the radio source consists of a series of knots and jet-like prot rusions, with a sharp bend, strongly suggesting that the jet propagati on is constrained by a dense external medium, Further evidence that 11 38-262 resides in a dense environment comes from the extremely large r otation measure (6200 rad m(-1)), the highest yet measured in a high r edshift radio galaxy. This suggest that the source is at the center of an hot X-ray emitting halo, possibly a massive cooling flow (similar to 500 M.yr(-1)). Finally, a small emission line galaxy is observed 7 '' to the north of the main galaxy: on the basis of both broad band an d narrow band images we conclude that it is al the same redshift as 11 38-262: therefore it is a further evidence of the overdense environmen t of the radio galaxy. The bright K-band magnitude of 1138-262 suggest that the host of the radio source is a very massive galaxy, of the or der 10(12) M.. The K band morphology can be fitted well by a classical De Vaucouleurs profile, indicating that 1138-262 is a well formed ell iptical galaxy. However the UV/optical broad band morphology is very c lumpy, consisting of a central region and a number of knots with sizes 10 to 15 kpc, which are systematically bluer that the central emissio n. All these components are embedded in a giant (similar to 100 kpc) L y alpha emission gas halo, which is also very clumpy and shows a high velocity dispersion. We argue that in the outer regions star formation is still occurring, either triggered by the passage of the radio jets or by the tidal interactions between the clumps that are falling on t he parent galaxy. Our favored interpretation of the observations is th at 1138-262 is a massive elliptical galaxy, at the center of a dense, cluster-like region. The galaxy is still accreting mass from its envir onment, and win therefore become even more massive. This strongly supp orts the idea that 1138-262 will eventually become a cD galaxy.