The Argo Simulation: II. The Early Build-up of the Hubble Sequence
  The Hubble sequence is a common classification scheme for the structure of
galaxies. Despite the tremendous usefulness of this diagnostic, we still do not
fully understand when, where, and how this morphological ordering was put in
place. Here, we investigate the morphological evolution of a sample of 22 high
redshift ($z\geq3$) galaxies extracted from the Argo simulation. Argo is a
cosmological zoom-in simulation of a group-sized halo and its environment. It
adopts the same high resolution ($\sim10^4$ M$_\odot$, $\sim100$ pc) and
sub-grid physical model that was used in the Eris simulation but probes a
sub-volume almost ten times bigger with as many as 45 million gas and star
particles in the zoom-in region. Argo follows the early assembly of galaxies
with a broad range of stellar masses ($\log M_{\star}/{\rm M}_{\odot}\sim8-11$
at $z\simeq3$), while resolving properly their structural properties. We
recover a diversity of morphologies, including late-type/irregular disc
galaxies with flat rotation curves, spheroid dominated early-type discs, and a
massive elliptical galaxy, already established at $z\sim3$. We identify major
mergers as the main trigger for the formation of bulges and the steepening of
the circular velocity curves. Minor mergers and non-axisymmetric perturbations
(stellar bars) drive the bulge growth in some cases. The specific angular
momenta of the simulated disc components fairly match the values inferred from
nearby galaxies of similar $M_{\star}$ once the expected redshift evolution of
disc sizes is accounted for. We conclude that morphological transformations of
high redshift galaxies of intermediate mass are likely triggered by processes
similar to those at low redshift and result in an early build-up of the Hubble
sequence.
