Massive quiescent galaxies at $z\sim3$: a comparison of selection,
  stellar population and structural properties with simulation predictions
  We study stellar population and structural properties of massive
$\log(M_{\star} / M_{\odot}) > 11$ galaxies at $z\sim2.7$ in the Magneticum
(box 3) and IllustrisTNG (TNG100, TNG300) hydrodynamical simulations. We find
stellar mass functions broadly consistent with observations, with no scarcity
of massive, quiescent galaxies at $z\sim2.7$, but with a higher quiescent
galaxy fraction at high masses in IllustrisTNG. Average ages of simulated
quiescent galaxies are between 0.8 and 1.0 Gyr, older by a factor $\sim2$ than
observed in spectroscopically-confirmed quiescent galaxies at similar redshift.
Besides being potentially indicative of issues with star-formation recipes in
simulations, this discrepancy might also be partly explained by limitations in
the estimation of observed ages. We investigate the purity of simulated UVJ
rest-frame colorselected massive quiescent samples with photometric
uncertainties typical of deep surveys (e.g., COSMOS). We find evidence for
significant contamination (up to 60 percent) by dusty star-forming galaxies in
the UVJ region that is typically populated by older quiescent sources.
Furthermore, simulations suggest that the completeness of UVJ-selected
quiescent samples at this redshift may be reduced by 30 percent due to a high
fraction of young quiescent galaxies not entering the UVJ quiescent region.
Massive, quiescent galaxies in simulations have on average lower angular
momenta and higher projected axis ratios and concentrations than star-forming
counterparts. Average sizes of simulated quiescent galaxies are relatively
close to observed ones, and broadly consistent within the uncertainties. The
average size ratio of quiescent and star-forming galaxies in the probed mass
range is formally consistent with observations, although this result is partly
affected by poor statistics.
