Deep imaging of the shell elliptical galaxy NGC 3923 with MegaCam^⋆

¹ Astronomical Institute, Czech Academy of Sciences, Boční II 1401/1a, 14100 Prague, Czech Republic
e-mail: bilek@asu.cas.cz
² Faculty of Mathematics and Physics, Charles University in Prague, Ke Karlovu 3, 12116 Prague, Czech Republic
³ Laboratoire AIM Paris-Saclay, CEA/Irfu/SAp – CNRS – Université Paris Diderot, 91191 Gif-sur-Yvette Cedex, France ; and Observatoire de Paris, PSL Research University, France
⁴ Canadian Astronomy Data Centre, 5071 West Saanich Road, Victoria, BC, V9E 2E7, Canada
⁵ Department of Theoretical Physics and Astrophysics, Faculty of Science, Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic
⁶ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands

Received: 26 May 2015
Accepted: 24 November 2015

Abstract

Context. The elliptical galaxy NGC 3923 is known to be surrounded by a number of stellar shells, probable remnants of an accreted galaxy. Despite its uniqueness, the deepest images of its outskirts come from the 1980s. On the basis of the modified Newtonian dynamics (MOND), it has recently been predicted that a new shell lies in this region.

Aims. We obtain the deepest image ever of the galaxy, map the tidal features in it, and search for the predicted shell.

Methods. The image of the galaxy was taken by the MegaCam camera at the Canada-France-Hawaii Telescope in the g′-band. It reached the surface-brightness limit of 29 mag arcsec^-2. In addition, we reanalyzed an archival HST image of the galaxy.

Results. We detected up to 42 shells in NGC 3923. This is by far the highest number among all shell galaxies. We present the description of the shells and other tidal features in the galaxy. A probable progenitor of some of these features was discovered. The shell system likely originates from two or more progenitors. The predicted shell was not detected, but the new image revealed that the prediction was based on incorrect assumptions and poor data.