The two formation pathways of S0 galaxies
  Despite their ubiquity throughout the Universe, the formation of S0 galaxies
remains uncertain. Recent observations have revealed that S0 galaxies make up a
diverse population which is difficult to explain with a single formation
pathway, suggesting that the picture of how these galaxies form is more
complicated than originally envisioned. Here we take advantage of the latest
hydrodynamical cosmological simulations and follow up these studies with an
investigation into the formation histories of S0s in IllustrisTNG. We first
classify IllustrisTNG galaxies in a way which is fully consistent with the
observations, and reproduce the observed photometric and environmental
distributions seen for the S0 population. We then trace the formation histories
of S0 galaxies back through time, identifying two main distinct pathways; those
which experienced gas stripping via group infalls (37 percent of S0s) or
significant merger events (57 percent). We find that those forming via mergers
feature a transient star-forming ring, whose present-day occurrence rate
matches observations. We find that these formation pathways together can
reproduce the range in rotational support in observed S0s, concluding that
there are two main formation pathways for S0 galaxies.
