IQ Collaboratory III: The Empirical Dust Attenuation Framework -- Taking
  Hydrodynamical Simulations with a Grain of Dust
  We present the Empirical Dust Attenuation (EDA) framework -- a flexible
prescription for assigning realistic dust attenuation to simulated galaxies
based on their physical properties. We use the EDA to forward model synthetic
observations for three state-of-the-art large-scale cosmological hydrodynamical
simulations: SIMBA, IllustrisTNG, and EAGLE. We then compare the optical and UV
color-magnitude relations, $(g-r) - M_r$ and $(FUV-NUV)-M_r$, of the
simulations to a $M_r < -20$ and UV complete SDSS galaxy sample using
likelihood-free inference. Without dust, none of the simulations match
observations, as expected. With the EDA, however, we can reproduce the observed
color-magnitude with all three simulations. Furthermore, the attenuation curves
predicted by our dust prescription are in good agreement with the observed
attenuation-slope relations and attenuation curves of star-forming galaxies.
However, the EDA does not predict star-forming galaxies with low $A_V$ since
simulated star-forming galaxies are intrinsically much brighter than
observations. Additionally, the EDA provides, for the first time, predictions
on the attenuation curves of quiescent galaxies, which are challenging to
measure observationally. Simulated quiescent galaxies require shallower
attenuation curves with lower amplitude than star-forming galaxies. The EDA,
combined with forward modeling, provides an effective approach for shedding
light on dust in galaxies and probing hydrodynamical simulations. This work
also illustrates a major limitation in comparing galaxy formation models: by
adjusting dust attenuation, simulations that predict significantly different
galaxy populations can reproduce the same UV and optical observations.
