New Horizon: On the origin of the stellar disk and spheroid of field
  galaxies at $z=0.7$
  The origin of the disk and spheroid of galaxies has been a key open question
in understanding their morphology. Using the high-resolution cosmological
simulation, New Horizon, we explore kinematically decomposed disk and
spheroidal components of 144 field galaxies with masses greater than $\rm
10^9\,M_{\odot}$ at $z=0.7$. The origins of stellar particles are classified
according to their birthplace (in situ or ex situ) and their orbits at birth.
Before disk settling, stars form mainly through chaotic mergers between
proto-galaxies and become part of the spheroidal component. When disk settling
starts, we find that more massive galaxies begin to form disk stars from
earlier epochs; massive galaxies commence to develop their disks at $z\sim1-2$,
while low-mass galaxies do after $z\sim1$. The formation of disks is affected
by accretion as well, as mergers can trigger gas turbulence or induce
misaligned gas infall that prevents galaxies from forming co-rotating disk
stars. The importance of accreted stars is greater in more massive galaxies,
especially in developing massive spheroids. A significant fraction of the
spheroids comes from the disk stars that are perturbed, which becomes more
important at lower redshifts. Some ($\sim12.5\%$) of our massive galaxies
develop counter-rotating disks from the gas infall misaligned with the existing
disk plane, which can last for more than a Gyr until they become the dominant
component, and flip the angular momentum of the galaxy in the opposite
direction. The final disk-to-total ratio of a galaxy needs to be understood in
relation to its stellar mass and accretion history. We quantify the
significance of the stars with different origins and provide them as guiding
values.
