MOND prediction of a new giant shell in the elliptical galaxy NGC 3923
Context. Stellar shells, which form axially symmetric systems
of arcs in some elliptical galaxies, are most likely remnants
of radial minor mergers. They are observed up a radius of
similar to 100 kpc. The stars in them oscillate in radial
orbits. The radius of a shell depends on the free-fall time at
the position of the shell and on the time since the merger. We
previously verified the consistency of shell radii in the
elliptical galaxy NGC 3923 with its most probable MOND
potential. Our results implied that an as yet undiscovered
shell exists at the outskirts of the galaxy. Aims. We here
extend our study by assuming more general models for the
gravitational potential to verify the prediction of the new
shell and to estimate its position. Methods. We tested the
consistency of the shell radial distribution observed in NGC
3923 with a wide variety of MOND potentials of the galaxy. The
potentials differed in the mass-to-light ratio and in distance
to the galaxy. We considered different MOND interpolation
functions, values of the acceleration constant a(0), and
density profiles of the galaxy. We verified the functionality
of our code on a Newtonian self-consistent simulation of the
formation of a shell galaxy. Results. Our method reliably
predicts that exactly one new outermost shell exists at a
galactocentric radius of about 1900 '' (similar to 210 kpc) on
the southwestern side of the galaxy. Its estimated surface
brightness is about 28 mag arcsec(-2) in B - a value accessible
by current instruments. This prediction enables a rare test of
MOND in an elliptical down to an acceleration of a(0)/10. The
predictive power of our method is verified by reconstructing
the position of the largest known shell from the distribution
of the remaining shells.