Identification of active galaxies behind the Coma cluster of galaxies^*

¹ UMR 7550 CNRS, Observatoire Astronomique de Strasbourg, 11 rue de l'Université, F–67000 Strasbourg, France
² Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK
³ GRAAL, Université Montpellier II, F–34095 Montpellier Cedex 5, France
⁴ Hamburger Sternwarte, Gojenbergsweg 112, D–21029 Hamburg, Germany
⁵ Institut d'Astrophysique de Paris - CNRS, 98bis Boulevard Arago, F–75014 Paris, France
⁶ DAEC, Observatoire de Paris-Meudon, F–92195 Meudon Principal Cedex, France
⁷ Laboratoire d'Astronomie de l'Université Lille I, Impasse de l'Observatoire, F–59000 Lille, France
⁸ Centre d'Analyse des Images, Observatoire de Paris, 61 avenue de l'Observatoire, F–75014 Paris, France

Send offprint request to: C. Ledoux

Received: 12 January 1999
Accepted: 21 May 1999

Abstract

We describe the results of an efficient survey to identify bright active galactic nuclei (AGN) within (4.7 h^-1 Mpc projected radius) of the centre of the Coma cluster of galaxies. The candidates have been carefully selected either as unresolved UV-excess objects with no detected proper motion, or from identifications on objective-prism plates including a few ROSAT sources. A global success rate larger than 30% is achieved. Low-resolution spectra are presented for the extragalactic objects identified in the course of the survey: the sample contains 16 emission-line objects, a BL Lac candidate and an elliptical galaxy. The AGNs of this list are unique targets to study the association of the Lyman-α forest with large-scale structures and galactic haloes at low redshift, to search for warm gas within the Coma cluster and nearby filaments, and for optical monitoring to detect the micro-lensing variability produced by the baryonic dark matter in the cluster.