Deep imaging of the shell elliptical galaxy NGC3923 with MegaCam

TL;DR

This study used ultra-deep imaging to map the extensive shell system of galaxy NGC 3923, revealing 42 shells and challenging previous predictions about a new shell based on MOND, thereby providing new insights into its tidal features.

Contribution

It provides the deepest image of NGC 3923, detecting the highest number of shells, and critically evaluates prior shell predictions, showing they were based on incorrect assumptions.

Findings

Detected 42 shells, the most in any shell galaxy.

Discovered a probable progenitor of some shells.

Failed to find the predicted shell, refuting earlier predictions.

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/arcsec2. 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.