3D intrinsic shapes of quiescent galaxies in observations and
  simulations
  We study the intrinsic 3D shapes of quiescent galaxies over the last half of
cosmic history based on their axial ratio distribution. To this end, we
construct a sample of unprecedented size, exploiting multi-wavelength
$u$-to-$K_s$ photometry from the deep wide area surveys KiDS+VIKING paired with
high-quality $i$-band imaging from HSC-SSP. Dependencies of the shapes on mass,
redshift, photometric bulge prominence and environment are considered. For
comparison, the intrinsic shapes of quenched galaxies in the IllustrisTNG
simulations are analyzed and contrasted to their formation history. We find
that over the full $0<z<0.9$ range, and in both simulations and observations,
spheroidal 3D shapes become more abundant at $M_* > 10^{11}\ M_{\odot}$, with
the effect being most pronounced at lower redshifts. In TNG, the most massive
galaxies feature the highest ex-situ stellar mass fractions, pointing to
violent relaxation via mergers as the mechanism responsible for their 3D shape
transformation. Larger differences between observed and simulated shapes are
found at low to intermediate masses. At any mass, the most spheroidal quiescent
galaxies in TNG feature the highest bulge mass fractions, and conversely
observed quiescent galaxies with the highest bulge-to-total ratios are found to
be intrinsically the roundest. Finally, we detect an environmental influence on
galaxy shape, at least at the highest masses, such that at fixed mass and
redshift quiescent galaxies tend to be rounder in denser environments.
