Evolution of Galaxy Shapes from Prolate to Oblate through Compaction
  Events
  We study the evolution of global shapes of galaxies using cosmological
simulations. The shapes refer to the components of dark matter (DM), stars and
gas at the stellar half-mass radius. Most galaxies undergo a characteristic
compaction event into a blue nugget at $z\sim2-4$, which marks the transition
from a DM-dominated central body to a self-gravitating baryonic core. We find
that in the high-$z$, DM-dominated phase, the stellar and DM systems tend to be
triaxial, preferentially prolate and mutually aligned. The elongation is
supported by an anisotropic velocity dispersion that originates from the
assembly of the galaxy along a dominant large-scale filament. We estimate that
torques by the dominant halo are capable of inducing the elongation of the
stellar system and its alignment with the halo. Then, in association with the
transition to self-gravity, small-pericenter orbits puff up and the DM and
stellar systems evolve into a more spherical and oblate configuration, aligned
with the gas disc and associated with rotation. This transition typically
occurs when the stellar mass is $\sim 10^9$ M$_\odot$ and the escape velocity
in the core is $\sim 100$ km s$^{-1}$, indicating that supernova feedback may
be effective in keeping the core DM-dominated and the system prolate. The early
elongated phase itself may be responsible for the compaction event, and the
transition to the oblate phase may be associated with the subsequent quenching
in the core.
