  Submillimetre galaxies (SMGs) have long posed a challenge for theorists, and
self-consistently reproducing the properties of the SMG population in a
large-volume cosmological hydrodynamical simulation has not yet been achieved.
We use a scaling relation derived from previous simulations plus radiative
transfer calculations to predict the submm flux densities of simulated SMGs
drawn from cosmological simulations from the Illustris and IllustrisTNG
projects based on the simulated galaxies' star formation rates (SFRs) and dust
masses and compare the predicted number counts with observations. We find that
the predicted SMG number counts based on IllustrisTNG are significantly less
than observed (more than 1 dex at $S_{850} \gtrsim 4$ mJy). The simulation from
the original Illustris project yields more SMGs than IllustrisTNG: the
predicted counts are consistent with those observed at both $S_{850} \lesssim
5$ mJy and $S_{850} \gtrsim 9$ mJy and only a factor of $\sim 2$ lower than
observed at intermediate flux densities. The redshift distribution of SMGs with
$S_{850} > 3$ mJy in IllustrisTNG is consistent with the observed distribution,
whereas the Illustris redshift distribution peaks at significantly lower
redshift (1.5 vs. 2.8). We demonstrate that IllustrisTNG hosts fewer SMGs than
Illustris because in the former, high-mass ($M_{\star} \sim 10^{11} \,
\text{M}_{\odot}$) $z \sim 2-3$ galaxies have lower dust masses and SFRs than
in Illustris owing to differences in the sub-grid models for stellar or/and
active galactic nucleus (AGN) feedback between the two simulations (we
unfortunately cannot isolate the specific cause(s) post hoc). Our results
demonstrate that because our method enables predicting SMG number counts in
post-processing with a negligible computational expense, SMGs can provide
useful constraints for tuning sub-grid models in future large-volume
cosmological simulations.
