The Nature of Post-Starburst Galaxies: Real Deal or Masquerading Impostors?

Post-starburst galaxies (PSBs) are a population of galaxies with spectral and photometric features indicative of rapid quenching following a recent starburst. The origin and nature of PSBs are currently debated. For example, a number of observed PSBs unexpectedly host substantial molecular gas despite their low inferred star-formation activity. Furthermore, the relative roles of galaxy interactions and quenching mechanisms in PSBs remain unclear. We study PSBs at $z=0.7$ and $z=1$ in the FIREbox cosmological simulation, selecting them primarily via their rest-frame optical photometric properties. The fraction of PSBs in FIREbox broadly agrees with observations, although some candidates are clear impostors with star-formation rates comparable to star-forming galaxies of similar mass. Impostors are rich in molecular gas and have a larger near-to-mid infrared flux ratios compared to quenched PSBs in the sample. The role of galaxy interactions of PSBs in FIREbox depends on their stellar mass. At low stellar masses ($\lesssim 10^{10}~\mathrm{M}_\odot$), PSBs have interaction fractions comparable to those of non-PSBs in the simulation, consistent with a scenario in which stellar feedback and gas consumption drive temporary quenching of star formation. At higher stellar masses ($\gtrsim 10^{10}~\mathrm{M}_\odot$), PSBs are preferentially interacting systems compared to non-PSBs, with major mergers providing the dominant contribution. We conclude that stellar feedback and galaxy interactions in FIREbox can produce galaxies with observational properties akin to those of observed PSBs, many of which are actively forming stars. Additional quenching channels, such as massive black hole feedback, are likely required to explain a long-lived, quenched population of PSBs.