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 disk 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_star < 10^{9.5} M_sun galaxies grow in a
disorganised 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_sun < M_star < 10^{10.5} M_sun galaxies
evolve towards a disk-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_star > 10^{10.5} M_sun growth through in-situ star
formation slows down considerably and galaxies transform towards a more
spheroidal morphology. This transformation is driven more by the buildup of
spheroids than by the destruction of disks. 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).
