Formation of slowly rotating early-type galaxies via major mergers: a
  Resolution Study
  We study resolution effects in numerical simulations of gas-rich and gas-poor
major mergers, and show that the formation of slowly-rotating elliptical
galaxies often requires a resolution that is beyond the present-day standards
to be properly modelled. Our sample of equal-mass merger models encompasses
various masses and spatial resolutions, ranging from about 200pc and 10^5
particles per component, typical of some recently published major merger
simulations, to up to 32pc and 10^3 M_sun in simulations using 2.4 x 10^7
collisionless particles and 1.2 x 10^7 gas particles, among the highest
resolutions reached so far for gas-rich major merger of massive disc galaxies.
We find that the formation of fast-rotating early-type galaxies, that are
flattened by a significant residual rotation, is overall correctly reproduced
at all such resolutions. However, the formation of slow-rotating early-type
galaxies, which have a low residual angular momentum and are supported mostly
by anisotropic velocity dispersions, is strongly resolution-dependent. The
evacuation of angular momentum from the main stellar body is largely missed at
standard resolution, and systems that should be slow rotators are then found to
be fast rotators. The effect is most important for gas-rich mergers, but is
also witnessed in mergers with an absent or modest gas component. The effect is
robust with respect to our initial conditions and interaction orbits, and
originates in the physical treatment of the relaxation process during the
coalescence of the galaxies. Our findings show that a high-enough resolution is
required to accurately model the global properties of merger remnants and the
evolution of their angular momentum. The role of gas-rich mergers of spiral
galaxies in the formation of slow-rotating ellipticals may therefore have been
underestimated.
