Evidence for multiple types of post-starburst galaxies

The quenching mechanisms of galaxies are not yet fully understood, but post-starburst galaxies provide one explanation for the rapid transition between star-forming and quiescent galaxies. At low redshift, it is generally thought that the starburst initiating the post-starburst phase is merger-driven, however, not all post-starburst galaxies show evidence of a merger, and recent studies suggested that post-starburst galaxies may be produced by multiple distinct mechanisms. This study examines whether multiple types of post-starburst galaxies actually exist, that is, whether the properties of post-starburst galaxies are multimodal. We used uniform manifold approximation and projection (UMAP) to cluster post-starburst galaxies based on spectroscopic data. The results suggest that there are three types of post-starburst galaxies that have dissimilar stacked spectral energy distributions and are separated by a combination of their Hα and [OII]λ3727 line strengths with an accuracy of 91%. A comparison of various galaxy properties (e.g., emission line strengths, mass and age distributions, and morphologies) indicates that the grouping is not just an age sequence, but may be correlated to the merger-histories of the galaxies. It suggested that the three post-starburst galaxy types have different origins, some of which may not be merger-driven, and that all typical galaxies go through the post-starburst phase at turnoff.