Times, environments and channels of bulge formation in a LambdaCDM cosmology

TL;DR

This study compares galaxy formation models within a LambdaCDM universe to understand bulge formation mechanisms, highlighting the roles of mergers, disc instability, and gas cooling in shaping galaxy morphology over time.

Contribution

It provides a comparative analysis of two galaxy formation models, emphasizing the impact of different bulge formation channels and their dependence on galaxy mass and environment.

Findings

Major and minor mergers significantly contribute to bulge mass.

Disc instability is crucial for intermediate mass galaxies.

Gas cooling influences the timing and channels of bulge formation.

Abstract

We analyze predictions from two independently developed galaxy formation models to study the mechanisms, environments, and characteristic times of bulge formation in a LambdaCDM cosmogony. For each model, we test different prescriptions for bulge formation in order to quantify the relative importance of different channels. Our results show that the strong correlation between galaxy and halo mass for central galaxies, and the richer merger history of more massive systems naturally give rise to a strong correlation between galaxy mass and morphology, and between halo mass and morphological type of central galaxies. Large fractions of the bulge mass are acquired through major and minor mergers, but disc instability plays an important role, particularly for intermediate mass galaxies. We find that the modelling of disc instability events, as well as of the galaxy merger times, can affect significantly the timing of bulge formation, and the relative importance of different channels. Bulge dominated galaxies acquire their morphology through major mergers, but this can be modified by cooling of gas from the surrounding hot halo. We find that disc regrowth is a non negligible component of the evolution of bulge dominated galaxies, particularly for low to intermediate masses, and at high redshifts.