The time-scales of major mergers from simulations of isolated binary
  galaxy collisions
  A six-dimensional parameter space based on high-resolution numerical
simulations of isolated binary galaxy collisions has been constructed to
investigate the dynamical friction timescales, $\tau_{\rm mer}$, for major
mergers. Our experiments follow the gravitational encounters between $\sim 600$
pairs of similarly massive late- and early-type galaxies with orbital
parameters compliant to the predictions of the LambdaCDM cosmology. We analyze
the performance of different schemes for tracking the secular evolution of
mergers, finding that the product of the intergalactic distance and velocity is
best suited to identify the time of coalescence. In contrast, a widely used
merger time estimator such as the exhaustion of the orbital spin is shown to
systematically underpredict $\tau_ {\rm mer}$, resulting in relative errors
that can reach 60% for nearly radial encounters. Regarding the internal spins
of the progenitors, we find that they can lead to total variations in the
merger times above 30% in highly circular encounters, whereas only that of the
principal halo is capable of modulating the strength of the interaction
prevailing throughout a merger. The comparison of our simulated merger times
with predictions from different variants of a well-known fitting formula has
revealed an only partially satisfactory agreement, which has led us to
recalculate the values of the coefficients of these expressions to obtain
relations that fit perfectly major mergers. The observed biases between data
and predictions, that do not only apply to the present work, are inconsistent
with expectations from differences in the degree of idealization of the
collisions, their metric, spin-related biases, or the simulation set-up. This
hints to a certain lack of accuracy of the dynamical friction modelling,
arising perhaps from a still not quite complete identification of the
parameters governing orbital decay.
