Radialization of satellite orbits in galaxy mergers

TL;DR

This study investigates how satellite orbits become more eccentric during galaxy mergers, revealing the importance of self-gravity and physical factors in orbital radialization through extensive N-body simulations.

Contribution

It demonstrates that satellite orbital radialization depends on self-gravity and physical effects, challenging classical dynamical friction models in galaxy merger simulations.

Findings

Radialization is most efficient with high satellite mass and initial eccentricity.

Self-gravity of host and satellite is crucial for orbital radialization.

Classical Chandrasekhar formula does not accurately predict eccentricity evolution.

Abstract

We consider the orbital evolution of satellites in galaxy mergers, focusing on the evolution of eccentricity. Using a large suite of N-body simulations, we study the phenomenon of satellite orbital radialization -- a profound increase in the eccentricity of its orbit as it decays under dynamical friction. While radialization is detected in a variety of different setups, it is most efficient in the cases of high satellite mass, not very steep host density profiles, and high initial eccentricity. To understand the origin of this phenomenon, we run additional simulations with various physical factors selectively turned off: satellite mass loss, reflex motion and distortion of the host, etc. We find that all these factors are important for radialization, since it does not occur for point-mass satellites or when the host potential is replaced with an unperturbed initial profile. The analysis of forces and torques acting on both galaxies confirms the major role of self-gravity of both host and satellite in the reduction of orbital angular momentum. The classical Chandrasekhar dynamical friction formula, which accounts only for the forces between the host and the satellite, but not for internal distortions of both galaxies, does not match the evolution of eccentricity observed in N-body simulations.