The star-formation activity of IllustrisTNG galaxies: main sequence, UVJ
  diagram, quenched fractions, and systematics
  We select galaxies from the IllustrisTNG hydrodynamical simulations
($M_*>10^9~\rm M_\odot$ at $0\le z\le2$) and characterize the shapes and
evolutions of their UVJ and star-formation rate -- stellar mass (SFR-$M_*$)
diagrams. We quantify the systematic uncertainties related to different
criteria to classify star-forming vs. quiescent galaxies, different SFR
estimates, and by accounting for the star formation measured within different
physical apertures. The TNG model returns the observed features of the UVJ
diagram at $z\leq2$, with a clear separation between two classes of galaxies.
It also returns a tight star-forming main sequence (MS) for $M_*<10^{10.5}\,\rm
M_\odot$ with a $\sim0.3$ dex scatter at $z\sim0$ in our fiducial choices. If a
UVJ-based cut is adopted, the TNG MS exhibits a downwardly bending at stellar
masses of about $10^{10.5-10.7}~\rm M_\odot$. Moreover, the model predicts that
$\sim80\,(50)$ per cent of $10^{10.5-11}~\rm M_\odot$ galaxies at $z=0~(z=2)$
are quiescent and the numbers of quenched galaxies at intermediate redshifts
and high masses are in better agreement with observational estimates than
previous models. However, shorter SFR-averaging timescales imply higher
normalizations and scatter of the MS, while smaller apertures lead to
underestimating the galaxy SFRs: overall we estimate the inspected systematic
uncertainties to sum up to about $0.2-0.3$ dex in the locus of the MS and to
about 15 percentage points in the quenched fractions. While TNG color
distributions are clearly bimodal, this is not the case for the SFR logarithmic
distributions in bins of stellar mass (SFR$\geq 10^{-3}~\rm M_\odot$yr$^{-1}$).
Finally, the slope and $z=0$ normalization of the TNG MS are consistent with
observational findings; however, the locus of the TNG MS remains lower by about
$0.2-0.5$ dex at $0.75\le z<2$ than the available observational estimates taken
at face value.
