From Discs to Bulges: effect of mergers on the morphology of galaxies
  We study the effect of mergers on the morphology of galaxies by means of the
simulated merger tree approach first proposed by Moster et al. This method
combines N-body cosmological simulations and semi-analytic techniques to
extract realistic initial conditions for galaxy mergers. These are then evolved
using high resolution hydrodynamical simulations, which include dark matter,
stars, cold gas in the disc and hot gas in the halo. We show that the satellite
mass accretion is not as effective as previously thought, as there is
substantial stellar stripping before the final merger. The fraction of stellar
disc mass transferred to the bulge is quite low, even in the case of a major
merger, mainly due to the dispersion of part of the stellar disc mass into the
halo. We confirm the findings of Hopkins et al., that a gas rich disc is able
to survive major mergers more efficiently. The enhanced star formation
associated with the merger is not localised to the bulge of galaxy, but a
substantial fraction takes place in the disc too. The inclusion of the hot gas
reservoir in the galaxy model contributes to reducing the efficiency of bulge
formation. Overall, our findings suggest that mergers are not as efficient as
previously thought in transforming discs into bulges. This possibly alleviates
some of the tensions between observations of bulgeless galaxies and the
hierarchical scenario for structure formation.
