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

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, rooted in the inherently stochastic nature of galactic star formation histories. Contrary to common interpretations in the literature, the SBMS phase marks a transition between starburst and more quiescent star formation in only a small fraction of cases (< 25%). Compaction events do raise the star formation rate, but do not reduce the gas depletion time, i.e., the timescale for star formation. We find no evidence linking the SBMS regime to quenching via compaction (i.e., the blue-then-red-nugget pathway). The importance of the SBMS regime and the variety of evolutionary tracks into and out of it challenges attempts to summarize the evolution of star-forming galaxies with a single, universal scenario.