Forming disc galaxies in major mergers II. The central mass
  concentration problem and a comparison of GADGET3 with GIZMO
  Context: In a series of papers, we study the major merger of two disk
galaxies in order to establish whether or not such a merger can produce a disc
galaxy. Aims: Our aim here is to describe in detail the technical aspects of
our numerical experiments. Methods: We discuss the initial conditions of our
major merger, which consist of two protogalaxies on a collision orbit. We show
that such merger simulations can produce a non-realistic central mass
concentration, and we propose simple, parametric, AGN-like feedback as a
solution to this problem. Our AGN-like feedback algorithm is very simple: at
each time-step we take all particles whose local volume density is above a
given threshold value and increase their temperature to a preset value. We also
compare the GADGET3 and GIZMO codes, by applying both of them to the same
initial conditions. Results: We show that the evolution of isolated
protogalaxies resembles the evolution of disk galaxies, thus arguing that our
protogalaxies are well suited for our merger simulations. We demonstrate that
the problem with the unphysical central mass concentration in our merger
simulations is further aggravated when we increase the resolution. We show that
our AGN-like feedback removes this non-physical central mass concentration, and
thus allows the formation of realistic bars. Note that our AGN-like feedback
mainly affects the central region of a model, without significantly modifying
the rest of the galaxy. We demonstrate that, in the context of our kind of
simulation, GADGET3 gives results which are very similar to those obtained with
the PSPH (density independent SPH) flavor of GIZMO. Moreover, in the examples
we tried, the differences between the results of the two flavors of GIZMO,
namely PSPH, and MFM (mesh-less algorithm) are similar to and, in some
comparisons, larger than the differences between the results of GADGET3 and
PSPH.
