The shell system around NGC 3923 and its implications for the potential of the galaxy
Résumé
A study of the shell galaxy NGC 3923 is presented. It is mainly based on CCD and photographic observations made at the CFHT, ESO, and AAT from 1982 to 1986. New shells have been discovered. The geometry of 22 shells is investigated in detail with an estimate of the uncertainties. The interleaving of the shells is not a clear as it has been claimed: it is well defined for the outer shells but the distances of the inner shells present a certain symmetry relative to the center. Shells have a roughly constant ellipticity of E = 1. +/- 0.4 [ E = 10 x ( 1 - b/a)]. It has been suggested by some authors that shell ellipticities are related to the elliptical potential; a brief study of current models of elliptical galaxies shows that the shell ellipticities are of the same magnitude as the equipotential ellipticities, but no clear relation id visible. Profiles were derived for 19 shells, and a simple model was fitted to these profiles. The shell thickness is typically of 0.17 kpc for the inner shells. The total B luminosity of the shell system is ~ 5 % (+/- 1 %) of the luminosity of the galaxy. The outermost shells are much more luminous than the inner ones: the outer four shells contribute ~85% of the total luminosity. A dust lane has been discovered on deep CCD exposures. If it is in a stable configuration its orientation is in contradiction with some previous studies based on numerical simulations which concluded that NGC 3923 was a prolate system. A photometric study of the galaxy is presented. The inferred central M/L ratio, is close to 10 which does not depart from the values usually found in ellipticals. Observations in the 21 cm hydrogen line with the Parkes Radio Telescope gives an upper limit of the H I content of 7 x 10^8 solar mass.
The results of the present study seem compatible with the merging hypothesis but dynamical friction has to be invoked to account for the presence of the innermost shells we have observed.