Observations of clusters at high redshift () can directly constrain cosmological models (e.g. Bahcall & Fan 1998), but searches based on colours or narrow band emission have not established the presence of massive clusters (Le Fèvre et al. 1996; Pascarelle et al. 1996; Keel et al. 1999). There are several indications (e.g. Pentericci et al. 1999) that powerful radio galaxies at high redshift (HzRGs) tend to be in the center of forming clusters. The powerful radio galaxy PKS 1138-262 at redshift 2.156 is a prime example of a forming brightest cluster galaxy and has extensively been studied (e.g. Pentericci et al. 1997). The arguments for 1138-262 being at the center of a cluster include (a) the very clumpy morphology as observed by the HST (Pentericci et al. 1998), reminiscent of a massive merging system; (b) the extremely distorted radio morphology and the detection of the largest radio rotation measures (6200 rad m-2) in a sample of more than 70 HzRGs, indicating that 1138-262 is surrounded by a hot, clumpy and dense magnetized medium (Carilli et al. 1997; Pentericci et al. 2000); (c) the detection of X-ray emission around 1138-262 (Carilli et al. 1998), indicating the presence of hot cluster gas, although a contribution to the X-ray luminosity by the AGN cannot be precluded. For this reason, we chose 1138-262 to carry out a pilot study with the VLT, to search for direct evidence of clusters at high redshift. There are various techniques for detecting high redshift companion galaxies. The colour selection technique used to find Lyman Break Galaxies (LBGs) (Steidel & Hamilton 1992) is not feasible at the redshift of 1138-262, since the Lyman limit falls at 2878 Å, which is well below the atmospheric cutoff. Therefore we have adopted the strategy of narrow band imaging at the wavelength of the redshifted Ly line. This technique is capable of detecting galaxies at redshifts similar to the radio galaxy redshift having strong Ly emission.
© European Southern Observatory (ESO) 2000
Online publication: June 26, 2000