Universal bimodality in kinematic morphology and the divergent pathways to galaxy quenching

TL;DR

This study reveals a universal bimodal distribution of galaxy kinematic morphology linked to star formation activity, indicating two distinct evolutionary pathways to galaxy quenching across all environments.

Contribution

It uncovers a universal bimodality in galaxy spin parameters, showing different quenching pathways for cold disc and hot spheroid populations.

Findings

Bimodal distribution of intrinsic spin parameter in galaxies.

Distinct star formation and metal enrichment histories for the two populations.

Bimodality persists across all environments and star formation states.

Abstract

The hierarchical structure formation of our Universe inherently involves violent and chaotic episodes of mass assembly such as galaxy mergers. The level of bulk rotation of the collisionless stellar systems of galaxies reflects to what extent the galaxies, on the other hand, have assembled their stars during tranquil and ordered formation history, which fosters the growth of cohesively rotating structures. Observationally, galaxy populations show a wide spectrum of morphology and shapes, with different levels of rotational support. Despite the obvious variety and complexity, in this work we find that at a given stellar mass of galaxies, the distribution of the intrinsic spin parameter $\lambda_{R_{\rm e},\mathrm{intr}}$, i.e. the normalized specific angular momentum of stars, appears to be universally bimodal among galaxies in all star formation states and also in different environments. This ubiquitous bimodality in kinematic morphology evolves systematically with star formation and is particularly apparent for transitional galaxies of intermediate star formation rates, indicating that star formation quenching is proceeding separately within two distinct kinematic populations dominated by cold discs and hot spheroids. We show that the two populations also have contrasting recent star formation histories and metal enrichment histories, which reveal their divergent pathways to formation and quenching.