Merging of Elliptical Galaxies as a Possible Origin of the Intergalactic Stellar Population

We present N-body simulations of elliptical galaxy encounters into dry mergers to study the resulting unbound intergalactic stellar population, in particular that of the post-main-sequence stars. The systems studied are pairs of spherical galaxies without dark halos. The stellar content of the model galaxies is distributed into mass bins representing low- and intermediate-mass stars (0.85-8 M_☉) according to the Salpeter initial mass function. Our models follow the dynamical evolution of galaxy encounters colliding head-on from initial low-energy parabolic or high-energy mildly hyperbolic orbits and for a choice of initial mass ratios. The merging models with initial parabolic orbits have M₂/M₁ = 1 and 10, and they leave behind, respectively, 5.5% and 10% of the total initial mass as unbound stellar mass. The merging model with an initial hyperbolic orbit has M₂/M₁ = 1 and leaves behind 21% of its initial stellar mass as unbound mass, showing that the efficiency in producing intergalactic stars through a high-energy hyperbolic encounter is about 4 times that of a parabolic encounter of the same initial mass ratio. By assuming that all progenitor galaxies, as well as the merger remnants, are homologous systems we find that the intergalactic starlight is 17% and 28% of the total starlight, respectively, for the parabolic and hyperbolic encounters with M₂/M₁ = 1. In all models, stars of different mass have the same probability of becoming unbound and feeding the intergalactic stellar population.