Galactic mergers and the persistence of the core Fundamental Plane.

Abstract

The destruction of dense elliptical galaxies during galactic mergers is studied through numerical simulations. The initial galaxy structure was represented as an eta model, where the parameters were chosen to be consistent with the core and global Fundamental Plane. Mergers with mass ratios of up to 100 and with central density ratios of up to 1000 were simulated. The simulations were conducted in two sets. In the first set, each galaxy is composed only of stars. In the second set, the primary galaxy hosts a massive black hole. We utilized our own self-consistent particle-field code to advance the merger. Secondary galaxies are not destroyed in dissipationless, singularity free minor mergers, as long as the secondary central density is at every point greater than the primary density. Instead, the secondary experiences mass loss to approximately the Roche radius, and is substantially intact inside that radius. The secondary is destroyed only when the initial primary and secondary central densities are comparable. This result holds regardless of the initial secondary orbital configuration. The failure of these experiments to destroy dense secondaries produces a merger remnant with a much higher central density than is allowed on the core Fundamental Plane, and implies that mergers require more than just the dynamics of visible stars. The addition of a massive central singularity inside the primary galaxy provides a strong enough impulsive energy injection to destroy a dense secondary on elongated encounters. On more circular orbits, the secondary is only disrupted near the end of the merger, and the degree to which the secondary particles disperse depends on the amount of orbital energy left in the merger. Hence, there are some unphysically dense merger remnants resulting from mergers with high angular momentum, despite the existence of a central black hole. We find that if the secondary is not dispersed, it forms a spinning stellar disk with a central density that forces the merger remnant off the cFP. These thick stellar disks are suggestive of the disk in M31 in particular, the different metallicity of the stellar disk in M31 as compared to the galaxy as a whole can be interpreted by our experiments as arising from different stellar populations in the primary and secondary galaxies.