Dissipationless collapse and the dynamical mass–ellipticity relation of elliptical galaxies in Newtonian gravity and MOND

Context. Recent observational studies proposed an empirical relation between the dark-to-total mass ratio and ellipticity in elliptical galaxies based on their observed total dynamical mass-to-light ratio data M/L = (14.1 ± 5.4)Ïμ. In other words, the larger the content of dark matter in the galaxy, the more the stellar component will be flattened. If true, this observation appears to be in stark contrast with the commonly accepted galaxy formation scenario, whereby this process takes place inside dark halos with reasonably spherical symmetry. Aims. Comparing the processes of dissipationless galaxy formation in different theories of gravity and the emergence of the galaxy scaling relations therein provides an important framework within which, in principle, one can discriminate between these processes. Methods. By means of collisionless N-body simulations in modified Newtonian dynamics (MOND) and Newtonian gravity with and without active dark matter halos, with both spherical and clumpy initial structure, I study the trends of intrinsic and projected ellipticities, Sérsic index, and anisotropy with the total dynamical-to-stellar mass ratio. Results. I show that the end products of both cold spherical collapses and mergers of smaller clumps show an increasing departure from spherical symmetry for increasing values of the total dynamical-to-stellar mass ratio, at least in a range of halo masses. The equivalent Newtonian systems of the end products of MOND collapses show a similar behaviour. However, the M/L relation obtained from the numerical experiments in both gravities is rather different from that reported by Deur and coauthors.