  Forward-modeling observables from galaxy simulations enables direct
comparisons between theory and observations. To generate synthetic spectral
energy distributions (SEDs) that include dust absorption, re-emission, and
scattering, Monte Carlo radiative transfer is often used in post-processing on
a galaxy-by-galaxy basis. However, this is computationally expensive,
especially if one wants to make predictions for suites of many cosmological
simulations. To alleviate this computational burden, we have developed a
radiative transfer emulator using an artificial neural network (ANN),
ANNgelina, that can reliably predict SEDs of simulated galaxies using a small
number of integrated properties of the simulated galaxies: star formation rate,
stellar and dust masses, and mass-weighted metallicities of all star particles
and of only star particles with age <10 Myr. Here, we present the methodology
and quantify the accuracy of the predictions. We train the ANN on SEDs computed
for galaxies from the IllustrisTNG project's TNG50 cosmological
magnetohydrodynamical simulation. ANNgelina is able to predict the SEDs of
TNG50 galaxies in the ultraviolet (UV) to millimetre regime with a typical
median absolute error of ~7 per cent. The prediction error is the greatest in
the UV, possibly due to the viewing-angle dependence being greatest in this
wavelength regime. Our results demonstrate that our ANN-based emulator is a
promising computationally inexpensive alternative for forward-modeling galaxy
SEDs from cosmological simulations.
