Formation and Morphology of the First Galaxies in the Cosmic Morning
  We investigate the formation and morphological evolution of the first
galaxies in the cosmic morning ($10>z>4$) using the Horizon Run 5 (HR5)
cosmological hydrodynamical simulation. For galaxies above the stellar mass
$M_{\star} = 2\times10^9\,M_{\odot}$, we classify them into disk, spheroid, and
irregular types according to their asymmetry and stellar mass morphology. We
find that 2/3 of galaxies have a S\'{e}rsic index less than 1.5, reflecting the
dominance of disk-type morphology in the cosmic morning. The rest are evenly
distributed as irregulars or spheroids. We also find that these fractions are
roughly independent of redshift and stellar mass up to
$\sim10^{10}\,M_{\odot}$, while irregular or spheroidal morphology appears
incidental and transient. Almost all the first galaxies with
$M_{\star}>2\times10^9\,M_{\odot}$ at redshift 6 form at initial peaks of the
matter density field. Large-scale structures in the universe emerge and grow
like cosmic rhizomes as the underlying matter density fluctuations grow and
form associations of galaxies in rare overdense regions. The growth of the
density field further stretches the realm of the galactic world into relatively
lower-density regions along evolving filaments. The cosmic web of galaxies
forms at lower redshifts when most rhizomes globally percolate. The primordial
angular momentum produced by the induced tidal torques on protogalactic regions
is correlated with the internal kinematics of galaxies and tightly aligned with
the angular momentum of the total galaxy mass. However, the primordial angular
momentum only very weakly correlates with the instantaneous morphology and
orientation of the stellar component below $z=6$. The large-scale tidal field
imprinted in the initial conditions seems responsible for the dominance of disk
morphology, and for the tendency of galaxies to re-acquire a disk
post-distortion.
