Establish the relative importance of in-situ clump formation and merger-driven growth

Determine which mechanism, in-situ star-cluster formation or merger-driven hierarchical assembly enhanced by interaction-triggered star-forming bursts, contributes more to the early assembly of massive-galaxy progenitors.

Background

The paper contrasts two possible pathways for early galaxy growth: in-situ formation of massive star clusters or clumps within galaxies, and hierarchical assembly through galaxy mergers whose growth is accelerated by interaction-triggered bursts of star formation. The Building Blocks System provides evidence for the latter mechanism, while other strongly lensed high-redshift systems have been interpreted as examples of the former.

Because the available sample of systems is small, the authors explicitly leave unresolved which mechanism dominates the early assembly of present-day massive-galaxy progenitors. Larger samples and spatially resolved spectroscopy are identified as necessary to measure their relative contributions.

References

Given limited number statistics, we cannot conclude which mechanism (in-situ clump formation or burst-enhanced hierarchical assembly) dominates.

— Early steps in the hierarchical assembly of a Milky Way-mass galaxy 1 Gyr after the Big Bang  (2609.19687 - Asada et al., 17 Sep 2026) in Section 3.3, “In-situ growth vs. hierarchical assembly”; Section 4, “Conclusion”