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We study the effect of mergers on the morphology of galaxies by means of thesimulated merger tree approach first proposed by Moster et al. This methodcombines N-body cosmological simulations and semi-analytic techniques toextract realistic initial conditions for galaxy mergers. These are then evolvedusing high resolution hydrodynamical simulations, which include dark matter,stars, cold gas in the disc and hot gas in the halo. We show that the satellitemass accretion is not as effective as previously thought, as there issubstantial stellar stripping before the final merger. The fraction of stellardisc mass transferred to the bulge is quite low, even in the case of a majormerger, mainly due to the dispersion of part of the stellar disc mass into thehalo. We confirm the findings of Hopkins et al., that a gas rich disc is ableto survive major mergers more efficiently. The enhanced star formationassociated with the merger is not localised to the bulge of galaxy, but asubstantial fraction takes place in the disc too. The inclusion of the hot gasreservoir in the galaxy model contributes to reducing the efficiency of bulgeformation. Overall, our findings suggest that mergers are not as efficient aspreviously thought in transforming discs into bulges. This possibly alleviatessome of the tensions between observations of bulgeless galaxies and thehierarchical scenario for structure formation.

From Discs to Bulges: effect of mergers on the morphology of galaxies