THE EVOLUTION OF STELLAR VELOCITY DISPERSION DURING DISSIPATIONLESS GALAXY MERGERS

Using N-body simulations, we studied the detailed evolution of central stellar velocity dispersion, sigma(*), during dissipationless binary mergers of galaxies. Stellar velocity dispersion was measured using the common mass-weighting method as well as a flux-weighting method designed to simulate the technique used by observers. A toy model for dust attenuation was introduced in order to study the effect of dust attenuation on measurements of sigma(*). We found that there are three principal stages in the evolution of sigma(*) in such mergers: oscillation, phase mixing, and dynamical equilibrium. During the oscillation stage, sigma(*) undergoes damped oscillations of increasing frequency. The oscillation stage is followed by a phase mixing stage during which the amplitude of the variations in sigma(*) is smaller and more chaotic than in the oscillation stage. Upon reaching dynamical equilibrium, sigma(*) assumes a stable value. We used our data regarding the evolution of sigma(*) during mergers to characterize the scatter inherent in making measurements of sigma(*) in non-quiescent systems. In particular, we found that sigma(*) does not fall below 70% nor exceed 200% of its final, quiescent value during a merger and that a random measurement of sigma(*) in such a system is much more likely to fall near the equilibrium value than near an extremum. Our toy model of dust attenuation suggested that dust can systematically reduce observational measurements of sigma(*) and increase the scatter in sigma(*) measurements.