1 Sorbonne Universités, UPMC Univ. Paris 6 et CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98bis Bd Arago, 75014 Paris, France
2 Faculty of Engineering, Gifu University, 1-1 Yanagido, 501-1193 Gifu, Japan
3 Department of Astrophysics, Nagoya University, Furocho, Chikusaku, 464-8602 Nagoya, Japan
4 LAM, OAMP, Pôle de l’Étoile Site Château-Gombert, 38 rue Frédéric Joliot–Curie, 13388 Marseille Cedex 13, France
5 INAF/Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
Received: 14 May 2015
Accepted: 2 September 2015
Context. The Ophiuchus cluster, at a redshift z = 0.0296, is known from X-rays to be one of the most massive nearby clusters, but its optical properties have not been investigated in detail because of its very low Galactic latitude.
Aims. We discuss the optical properties of the galaxies in the Ophiuchus cluster, in particular, with the aim of understanding its dynamical properties better.
Methods. We have obtained deep optical imaging in several bands with various telescopes, and applied a sophisticated method to model and subtract the contributions of stars to measure galaxy magnitudes as accurately as possible. The colour−magnitude relations obtained show that there are hardly any blue galaxies in Ophiuchus (at least brighter than r′ ≤ 19.5), and this is confirmed by the fact that we only detect two galaxies in Hα. We also obtained a number of spectra with ESO-FORS2, which we combined with previously available redshifts. Altogether, we have 152 galaxies with spectroscopic redshifts in the 0.02 ≤ z ≤ 0.04 range, and 89 galaxies with both a redshift within the cluster redshift range and a measured r′ band magnitude (limited to the Megacam 1 × 1 deg2 field).
Results. A complete dynamical analysis based on the galaxy redshifts available shows that the overall cluster is relaxed and has a mass of 1.1 × 1015 M⊙. The Sernal-Gerbal method detects a main structure and a much smaller substructure, which are not separated in projection.
Conclusions. From its dynamical properties derived from optical data, the Ophiuchus cluster seems overall to be a relaxed structure, or at most a minor merger, though in X-rays the central region (radius ~ 150 kpc) may show evidence for merging effects.
Key words: galaxies: clusters: individual: Ophiuchus / galaxies: photometry
Based on observations obtained with MegaPrime/MegaCam (program 10AF02), a joint project of CFHT and CEA/DAPNIA, at the Canada-France-Hawaii Telescope (CFHT), which is operated by the National Research Council (NRC) of Canada, the Institut National des Sciences de l’Univers of the Centre National de la Recherche Scientifique of France, and the University of Hawaii. Based on observations performed with ESO Telescopes at the La Silla Paranal Observatory under programme ID 085.A-0016(C). Based on observations obtained at the Southern Astrophysical Research (SOAR) telescope (programme 2009B-0340 on SOI/SOAR), which is a joint project of the Ministério da Ciência, Tecnologia, e Inovação (MCTI) da República Federativa do Brasil, the US National Optical Astronomy Observatory (NOAO), the University of North Carolina at Chapel Hill (UNC), and Michigan State University (MSU). This research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration, and of the SIMBAD database, operated at CDS, Strasbourg, France.
Tables A.1–A.3 are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (184.108.40.206) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/583/A124
© ESO, 2015