From discs to bulges: effect of mergers on the morphology of galaxies

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.

On the dependence of galaxy morphologies on galaxy mergers

“…However, this is obtained at the expense of almost completely devoiding the B/T > 0.9 region. This effect goes into the same direction as for the HOP09 runs, but the overall reduction of bulge dominated galaxies is greater when using the Kannan et al (2015) prescriptions in these models: the KAN15 column of Fig. 1 shows that most model galaxies have B/T < 0.5.…”

“…In particular, the fraction of B/T > 0.7 galaxies, fE, is reduced at all mass scales. However, runs using the Kannan et al (2015) fitting formulae at all redshifts result in virtually no galaxy with B/T = 1. Moreover, in these model realizations most of the bulge mass is assembled at z > 1.…”

“…We briefly recall here the main results from Kannan et al (2015). Using hydrodynamical simulations based on the parallel TreeSPHcode GADGET-2 (Springel et al 2005), they show that in binary galaxy mergers, the mass exchanges between the satellite and the central galaxy and between the different components of the central galaxy primarly depend on the baryonic merger ratio (µ b ) between the two objects (computed at the infall time of the satellite galaxy, i.e.…”

“…In the following, we focus on two different sets of models: we first consider SAMs realizations (KAN15) where the Kannan et al (2015) fitting formulae are implemented at all redshifts. In a second set of modified models (Kz<1), we allow these modifications only for z < 1, keeping the reference merger prescriptions at earlier times.…”

The role of mergers and halo spin in shaping galaxy morphology

“…Khochfar & Silk 2006;Parry, Eke & Frenk 2009;De Lucia et al 2011;Zavala et al 2012;Avila-Reese, Zavala & Lacerna 2014;Fontanot et al 2015;Kannan et al 2015) or semiempirical halo occupation models (e.g. Hopkins et al 2009b;Stewart et al 2009;Hopkins et al 2010;Moster, Maccio & Somerville 2014).…”