The three phases of galaxy formation

We investigate the origin of the Hubble sequence by analysing the evolution of the kinematic morphologies of central galaxies in the EAGLE cosmological simulation. By separating each galaxy into disc and spheroidal stellar components and tracing their evolution along the merger tree, we find that the morphology of galaxies follows a common evolutionary trend. We distinguish three phases of galaxy formation. These phases are determined primarily by mass, rather than redshift. For M-* <= 10(9.5) M-circle dot galaxies grow in a disorganized way, resulting in a morphology that is dominated by random stellar motions. This phase is dominated by in situ star formation, partly triggered by mergers. In the mass range 10(9.5) M-circle dot <= M-* <= 10(10.5) M-circle dot, galaxies evolve towards a disc-dominated morphology, driven by in situ star formation. The central spheroid (i.e. the bulge) at z = 0 consists mostly of stars that formed in situ, yet the formation of the bulge is to a large degree associated with mergers. Finally, at M-* >= 10(10.5)M(circle dot) growth through in situ star formation slows down considerably and galaxies transform towards a more spheroidal morphology. This transformation is driven more by the build-up of spheroids than by the destruction of discs. Spheroid formation in these galaxies happens mostly by accretion at large radii of stars formed ex situ (i.e. the halo rather than the bulge).