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Context: In a series of papers, we study the major merger of two diskgalaxies in order to establish whether or not such a merger can produce a discgalaxy. Aims: Our aim here is to describe in detail the technical aspects ofour numerical experiments. Methods: We discuss the initial conditions of ourmajor merger, which consist of two protogalaxies on a collision orbit. We showthat such merger simulations can produce a non-realistic central massconcentration, and we propose simple, parametric, AGN-like feedback as asolution to this problem. Our AGN-like feedback algorithm is very simple: ateach time-step we take all particles whose local volume density is above agiven threshold value and increase their temperature to a preset value. We alsocompare the GADGET3 and GIZMO codes, by applying both of them to the sameinitial conditions. Results: We show that the evolution of isolatedprotogalaxies resembles the evolution of disk galaxies, thus arguing that ourprotogalaxies are well suited for our merger simulations. We demonstrate thatthe problem with the unphysical central mass concentration in our mergersimulations is further aggravated when we increase the resolution. We show thatour AGN-like feedback removes this non-physical central mass concentration, andthus allows the formation of realistic bars. Note that our AGN-like feedbackmainly affects the central region of a model, without significantly modifyingthe rest of the galaxy. We demonstrate that, in the context of our kind ofsimulation, GADGET3 gives results which are very similar to those obtained withthe PSPH (density independent SPH) flavor of GIZMO. Moreover, in the exampleswe tried, the differences between the results of the two flavors of GIZMO,namely PSPH, and MFM (mesh-less algorithm) are similar to and, in somecomparisons, larger than the differences between the results of GADGET3 andPSPH.

Forming disc galaxies in major mergers II. The central mass concentration problem and a comparison of GADGET3 with GIZMO