Galaxy interactions in IllustrisTNG-100, I: The power and limitations of
  visual identification
  We present a sample of 446 galaxy pairs constructed using the cosmological
simulation IllustrisTNG-100 at z = 0, with M$_{FoF, dm}$ =
10$^{11}$-10$^{13.5}$ M$_{\odot}$. We produce ideal mock SDSS g-band images of
all pairs to test the reliability of visual classification schema employed to
produce samples of interacting galaxies. We visually classify each image as
interacting or not based on the presence of a close neighbour, the presence of
stellar debris fields, disturbed discs, and/or tidal features. By inspecting
the trajectories of the pairs, we determine that these indicators correctly
identify interacting galaxies $\sim45\%$ of the time. We subsequently split the
sample into the visually identified interacting pairs (VIP; 38 pairs) and those
which are interacting but are not visually identified (nonVIP; 47 pairs). We
find that VIP have undergone a close passage nearly twice as recently as the
nonVIP, and typically have higher stellar masses. Further, the VIP sit in dark
matter haloes that are approximately 2.5 times as massive, in environments
nearly 2 times as dense, and are almost a factor of 10 more affected by the
tidal forces of their surroundings than the nonVIP. These factors conspire to
increase the observability of tidal features and disturbed morphologies, making
the VIP more likely to be identified. Thus, merger rate calculations which rely
on stellar morphologies are likely to be significantly biased toward massive
galaxy pairs which have recently undergone a close passage.
