Rapidly quenched galaxies in the Simba cosmological simulation and
  observations
  A wide range of mechanisms have been put forward to explain the quenching of
star formation in galaxies with cosmic time, however, the true balance of
responsible mechanisms remains unknown. The identification and study of
galaxies that have shut down their star formation on different timescales might
elucidate which mechanisms dominate at different epochs and masses. Here we
study the population of rapidly quenched galaxies (RQGs) in the SIMBA
cosmological hydrodynamic simulation at $0.5<z<2$, comparing directly to
observational post-starburst galaxies in the UKIDSS Ultra Deep Survey via their
colour distributions and mass functions. We find that the fraction of quiescent
galaxies that are rapidly quenched in SIMBA is 59% (or 48% in terms of stellar
mass), which is higher than observed. A similar "downsizing" of RQGs is
observed in both SIMBA and the UDS, with RQGs at higher redshift having a
higher average mass. However, SIMBA produces too many RQGs at $1<z_q<1.5$ and
too few low mass RQGs at $0.5<z_q<1$. The precise colour distribution of SIMBA
galaxies compared to the observations also indicates various inconsistencies in
star formation and chemical enrichment histories, including an absence of
short, intense starbursts. Our results will help inform the next generation of
galaxy evolution models, particularly with respect to the quenching mechanisms
employed.
