Astron. Nachr. / AN 327, No. 5/6, 571 – 572 (2006) / DOI 10.1002/asna.200610593 The radio luminosity function of radio galaxies in distant clusters M. Branchesi 1, , I.M. Gioia 1 , C. Fanti 1 , R. Fanti 1 , and R. Perley 2 1 INAF-Istituto di Radioastronomia, Bologna, Italy 2 NRAO, Socorro, NM, USA Received 2006 Feb 27, accepted 2006 Mar 21 Published online 2006 May 11 Key words galaxies: clusters – galaxies: high-redshift – radio continuum: galaxies A complete sample of 18 X-ray selected clusters of galaxies belonging to the ROSAT North Ecliptic Pole (NEP) survey has been observed with the Very Large Array at 1.4 GHz. These are the most distant clusters in the X-ray survey with redshift in the range 0.3 <z< 0.8. 79 radio sources are detected within half an Abell radius with an observed peak brightness ≥ 0.17 mJy beam −1 . 32 out of the 79 sources are within 0.2 Abell radius, 22 of them are considered cluster members based on spectroscopic redshifts or their optical magnitude and morphological classification. The cluster radio galaxies are used to construct the radio luminosity function (RLF) of distant X-ray selected clusters. A comparison with two nearby cluster RLFs shows that the NEP RLF lies above the local ones, has a steeper slope at low radio powers (≤ 10 24 W Hz −1 ) and shows no evidence for a break at ≈ 6 × 10 24 W Hz −1 which is observed in the nearby cluster RLFs. c  2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 Introduction The study of the effects produced by cluster formation on the properties of the radio sources embedded in the intra- cluster medium is one of the most fascinating topics in cluster astrophysics. Exhaustive and deep studies of opti- cal/radio properties of radio galaxies in nearby (z ≤ 0.25) Abell clusters have appeared in the literature: over 500 Abell clusters have been studied with the VLA by Owen, Ledlow, and collaborators (see Owen & Ledlow 1997, and references therein). An equivalent study for distant clusters has not yet been done even if radio surveys of a small num- ber of high-z, rich, X-ray selected clusters have been con- ducted (e.g. Stocke et al. 1999; Perlman et al. 2003), mainly with the goal of investigating the evolution of the cluster ra- dio galaxy population. According to the above authors an apparent dearth of radio sources below log P (W Hz -1 ) = 24.5 is due to incompleteness rather than to a weak negative evolution. We present here the results from a VLA survey which uses a new sample of X-ray selected, distant clusters of galaxies extracted from the ROSAT NEP survey (Gioia et al. 2003). Throughout the paper we assume H 0 = 75 km s -1 Mpc -1 and q 0 =0.1. The results obtained here would be strengthened if one adopts the currently-favored cosmological concordance model. 2 Radio observations and data reduction The sample contains 18 clusters which are the most dis- tant clusters in the NEP survey with redshifts in the range 0.3 <z< 0.8. Radio observations were carried out  Corresponding author: m.branchesi@ira.inaf.it at 1.4 GHz, using the Very Large Array in B-configuration. For each cluster one hour integration, divided into three “snapshot” scans, was obtained by pointing the array at the position of the brightest central galaxy. Each image mea- sures 25.6×25.6 arcmin. The final radio source sample con- tains 79 sources within half an Abell radius with an ob- served peak brightness S P ≥ 0.17 mJy beam -1 . 3 The NEP radio luminosity function The identification of a radio source with a cluster mem- ber has been made on the basis of the spectroscopic red- shift (available only for 14 galaxies belonging to 11 differ- ent clusters), of the absolute magnitude of the associated object, and on its optical classification on the SuperCOS- MOS/POSSII catalog. The radio luminosity function (RLF) for the NEP distant clusters (see Fig. 1) has been computed as the number of ra- dio galaxies per cluster as a fraction of radio power, and us- ing only those radio galaxies within 0.2 Abell radius where the field contamination is minimum. In order to correctly derive the RLF for the cluster radiogalaxies we had to eval- uate the contamination of field sources. Two methods were used, namely a statistical estimate of field sources and a best-guess allocation of cluster members on the basis of their optical properties. Both methods gave consistent re- sults for the non-cluster radio sources within 0.2 Abell radi us where the RLF is derived. Previous studies did not find any evidence for evolution in the population of distant cluster radio galaxies when compared to low redshift clusters (see Stocke et al. 1999; Perlman et al. 2003). In our X-ray selected sample a c  2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim