Quenching and the UVJ diagram in the SIMBA cosmological simulation
  Over the past decade, rest-frame color-color diagrams have become popular
tools for selecting quiescent galaxies at high redshift, breaking the color
degeneracy between quiescent and dust-reddened star-forming galaxies. In this
work, we study one such color-color selection tool -- the rest-frame $U-V$ vs.
$V-J$ diagram -- by employing mock observations of cosmological galaxy
formation simulations. In particular, we conduct numerical experiments
assessing both trends in galaxy properties in UVJ space and the color-color
evolution of massive galaxies as they quench at redshifts $z\sim 1$--$2$. We
find that our models broadly reproduce the observed UVJ diagram at $z=1$--$2$,
including (for the first time in a cosmological simulation) reproducing the
population of extremely dust-reddened galaxies in the top right of the UVJ
diagram. However, our models primarily populate this region with low-mass
galaxies and do not produce as clear a bimodality between star-forming and
quiescent galaxies as is seen in observations. The former issue is due to an
excess of dust in low-mass galaxies and relatively gray attenuation curves in
high-mass galaxies, while the latter is due to the overpopulation of the green
valley in SIMBA. When investigating the time evolution of galaxies on the UVJ
diagram, we find that the quenching pathway on the UVJ diagram is independent
of the quenching timescale, and instead dependent primarily on the average
specific star formation rate in the 1 Gyr prior to the onset of quenching. Our
results support the interpretation of different quenching pathways as
corresponding to the divergent evolution of post-starburst and green valley
galaxies.
