[Paper Review] Starbursts hiding in the main sequence: a pathway toward quenching?

F. Renaud, Katarina Kraljic+11 more|arXiv (Cornell University)|Feb 26, 2026

Galaxies: Formation, Evolution, Phenomena0 citations

TL;DR

The paper uses the NewHorizon cosmological simulation to define and study a population of SBMSs—starbursts lying within the star-forming main sequence—and analyzes their origins, evolution, and implications for galaxy evolution.

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]

Motivation & Objective

• Clarify the definitions and physical meaning of star formation regimes in galaxies, particularly SBMSs, to understand their origins and evolutionary role.
• Investigate how SBMSs arise in a cosmological context and whether they connect to compaction, mergers, or quenching pathways.
• Assess how SBMSs differ from classical starbursts and from main sequence galaxies in terms of gas content, size, and stellar assembly history.

Proposed method

• Employ the large-scale zoom-in cosmological NewHorizon simulation with high resolution (~34 pc) to model star formation, feedback, and black hole activity.
• Classify galaxies into four regimes (main sequence, starburst, SBMS, starflood) based on depletion time and offset from the main sequence.
• Use robust statistics (medians and median absolute deviation) to analyze population trends across redshift.
• Compute quantities within twice the stellar half-mass radius and use stars younger than 10 Myr for SFR-related metrics.
• Analyze structural and gas properties to infer the physical origin of SBMSs, including stellar mass assembly history and gas content.

Experimental results

Research questions

• RQ1 What defines SBMSs in a cosmological, model-consistent way, and how do they relate to the traditional main sequence and starburst classifications?
• RQ2 What are the physical conditions and evolutionary histories that lead to SBMS episodes, and do SBMSs imply a quenching pathway or a transitional regime?
• RQ3 How do SBMSs differ in gas fraction, star-forming gas size, and stellar compactness compared to main sequence and starburst galaxies across cosmic time?
• RQ4 Is SBMS behavior tied to compaction events or mergers, or does it arise from stochastic star formation histories?
• RQ5 Do SBMSs constitute a transitional phase toward quenching or a distinct, recurrent mode across redshift?

Key findings

• SBMSs are commonly found toward the high-mass end, with unusually short depletion times but located near the main sequence.
• SBMS galaxies have smaller star-forming gas radii and significantly lower gas fractions than analogues with similar M* and SFR, suggesting a precocious stellar mass assembly.
• SBMSs are more compact in both stellar and dense gas components, indicating a dynamical origin rather than purely hydrodynamical gas compaction.
• The SBMS regime is intermittent and short-lived (~30 Myr) and exists across cosmic time, driven by stochastic star formation histories.
• Overall, SBMSs do not strongly support a simple “blue nugget” compaction quenching pathway; there is no clear link between SBMSs and rapid quenching via compaction in the simulation.
• The fraction of star-forming galaxies that experience SBMS episodes increases with time, and SBMSs are not simply the high-mass tail of off-main-sequence starbursts.