Orientations of very faint galaxies in the Coma cluster^*

¹ LAM, Pôle de l'Étoile Site de Château-Gombert, 38 rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France e-mail: christophe.adami@oamp.fr
² Institut d'Astrophysique de Paris, CNRS, UMR 7095, Université Pierre et Marie Curie, 98bis Bd Arago, 75014 Paris, France
³ Canada-France-Hawaii Telescope Corporation, Kamuela, HI 96743, USA
⁴ Laboratoire d'Astrophysique de Toulouse-Tarbes, Université de Toulouse, CNRS, 14 Av. Edouard Belin, 31400 Toulouse, France
⁵ Department of Physics and Astronomy, Northwestern University, 2131 Sheridan Road, Evanston IL 60208-2900, USA

Received: 7 July 2008
Accepted: 2 October 2008

Abstract

Context. Models of large scale structure formation predict the existence of preferential orientations for galaxies in clusters.

Aims. We have searched for preferential orientations of very faint galaxies in the Coma cluster (down ).

Methods. By applying a deconvolution method to deep u^* and I band images of the Coma cluster, we were able to recover orientations down to very faint magnitudes, close to the faintest dwarf galaxies in the Coma cluster.

Results. No preferential orientations are found in more than 95% of the cluster area, and the brighter the galaxies, the fewer preferential orientations we detect. The minor axes of late type galaxies are radially oriented along a northeast-southwest direction and are oriented in a north-south direction in the western X-ray substructures. For early type galaxies in the western regions showing significant preferential orientations, galaxy major axes tend to be oriented perpendicularly to the north-south direction. In the eastern significant region and close to NGC 4889, galaxy major axes also tend to point toward the two cluster dominant galaxies. In the southern significant regions, galaxy planes tend to be tangential with respect to the clustercentric direction, except close to (, ) where the orientation is close to -15 deg. Early and late type galaxies do not have the same behaviour regarding orientation.

Conclusions. Considering various models, we give an interpretation which can account for the existence of preferential orientations for galaxies (e.g. the tidal torque model or interactions with the intra cluster medium). Part of the orientations of the minor axes of late type galaxies and of the major axes of early type galaxies can be explained by the tidal torque model applied both to cosmological filaments and local merging directions. Another part (close to NGC 4889) can be accounted for by collimated infalls. For early type galaxies, an additional region () shows orientations that probably result from local processes involving induced star formation.