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

TL;DR

This study uses advanced simulations to show that galaxy mergers are less effective at transforming discs into bulges than previously believed, especially considering stellar stripping and gas effects.

Contribution

It introduces a combined N-body and hydrodynamical simulation approach to assess the impact of mergers on galaxy morphology with more realistic initial conditions.

Findings

Stellar stripping reduces stellar mass transfer during mergers.

Gas-rich discs survive major mergers more effectively.

Hot gas reservoirs decrease bulge formation efficiency.

Abstract

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.