The radio galaxy 1138−262 at z=2.2: a giant elliptical 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 objectwas selected fromour compendiumof ultra steep spectrum radio sources on the basis of its distorted radio morphology. High resolution VLA radio observations show that the radio source consists of a series of knots and jet-like protrusions, with a sharp bend, strongly suggesting that the jet propagation is constrained by a dense external medium. Further evidence that 1138-262 resides in a dense environment comes from the extremely large rotation measure (6200 rad m−1), the highest yet measured in a high redshift radio galaxy. This suggest that the source is at the center of an hot X-ray emitting halo, possibly a massive cooling flow (∼ 500M 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 and narrow band images we conclude that it is at the same redshift as 1138−262; therefore it is a further evidence of the overdense environment 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 order 1012M . The K-bandmorphology can be fitted well by a classical De Vaucouleurs profile, indicating that 1138−262 is a well formed elliptical galaxy. However the UV/optical broad band morphology is very clumpy, consisting of a central region and a number of knots with sizes 10 to 15 kpc, which are systematically bluer that the central emission. All these components are embedded in a giant (∼ 100 kpc) Lyα 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 the parent galaxy. Our favored interpretation of the observations is that 1138−262 is amassive elliptical galaxy, at the center of a dense, cluster-like region. The galaxy is still accreting mass from its environment, and will therefore become even more massive. Send offprint requests to: L. Pentericci This strongly supports the idea that 1138−262 will eventually become a cD galaxy.