Starbursts hiding in the main sequence: a pathway toward quenching?

TL;DR

This paper investigates 'starbursts in the main sequence' galaxies using cosmological simulations, revealing their origins, intermittent nature, and role in galaxy evolution, especially in massive galaxies with frequent mergers.

Contribution

It identifies and characterizes a transient starburst regime within the main sequence, driven by mergers and early assembly, enhancing understanding of galaxy evolution pathways.

Findings

SBMSs are linked to early stellar mass assembly and mergers.

SBMS episodes are short-lived (~30 Myr) but occur across cosmic time.

Most SBMSs are found in the most massive galaxies.

Abstract

Star-forming galaxies spend most of their lifetimes on the star-forming main sequence, which establishes a tight empirical and statistical relation between stellar mass and star-formation rate. Occasional episodes of rapid star formation can push them temporarily above this sequence, turning them into starbursts. Yet some galaxies display starburst-like traits -- rapid, dense, and compact star formation -- while still remaining within the scatter of the main sequence. These "starbursts in the main sequence" (SBMSs) reveal the complexity and diversity of star formation modes, making them crucial for understanding how galaxies evolve and transition between different regimes. In this paper, we identify SBMSs in the cosmological simulation NewHorizon and follow their evolution across time to uncover their physical origins and the role of this special regime in shaping galaxy evolution. We explain the existence of SBMSs by a comparatively earlier assembly of their stellar mass, driven in particular by more frequent and repeated mergers as the other galaxies, as well as exceptionally productive starburst events triggered by these interactions. As a result, this regime appears preferentially -- though not exclusively -- in the most massive galaxies. The SBMS behavior is not continuous within individual galaxies but instead arises intermittently as a short-lived (~ 30 Myr) evolutionary mode. Nevertheless, such SBMS episodes exist throughout cosmic time across the galaxy population... [abridged]