IQ Collaboratory II: The Quiescent Fraction of Isolated, Low Mass
  Galaxies Across Simulations and Observations
  We compare three major large-scale hydrodynamical galaxy simulations (EAGLE,
Illustris-TNG, and SIMBA) by forward modeling simulated galaxies into
observational space and computing the fraction of isolated and quiescent low
mass galaxies as a function of stellar mass. Using SDSS as our observational
template, we create mock surveys and synthetic spectroscopic and photometric
observations of each simulation, adding realistic noise and observational
limits. All three simulations show a decrease in the number of quiescent,
isolated galaxies in the mass range $\mathrm{M}_* = 10^{9-10} \
\mathrm{M}_\odot$, in broad agreement with observations. However, even after
accounting for observational and selection biases, none of the simulations
reproduce the observed absence of quiescent field galaxies below
$\mathrm{M}_*=10^{9} \ \mathrm{M}_\odot$. We find that the low mass quiescent
populations selected via synthetic observations have consistent quenching
timescales, despite apparent variation in the late time star formation
histories. The effect of increased numerical resolution is not uniform across
simulations and cannot fully mitigate the differences between the simulations
and the observations. The framework presented here demonstrates a path towards
more robust and accurate comparisons between theoretical simulations and galaxy
survey observations, while the quenching threshold serves as a sensitive probe
of feedback implementations.
