The two formation pathways of S0 galaxies

TL;DR

This study uses cosmological simulations to identify two main formation pathways for S0 galaxies: gas stripping through group infalls and significant mergers, explaining their diverse properties.

Contribution

The paper demonstrates, through simulations, that S0 galaxies form via two distinct pathways, aligning with observed distributions and features.

Findings

37% of S0s formed via gas stripping from group infalls

57% of S0s formed through significant mergers

Transient star-forming rings observed in merger-formed S0s

Abstract

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.