Open Question on the Physical Origins of Massive Early-Universe Systems

Determine the physical origins of massive systems observed in the early Universe—including UV-bright galaxies at redshift z ≳ 9, broad-line massive active galactic nuclei at z ≈ 8.5, dusty massive starbursts at z ≈ 6.9, and overdensities of massive dusty galaxies at z ≈ 4.3—by isolating the roles of star-formation efficiency, feedback, and dark matter assembly.

Background

The report notes that recent JWST and ALMA observations have revealed numerous massive systems at high redshift whose properties are difficult to reconcile with current galaxy formation models. This motivates a focused investigation into their physical origins and the interplay of star-formation efficiency, feedback processes, and dark matter assembly.

Resolving this open question is central to understanding early galaxy ecosystems and informs the facilities and observational strategies needed, including ngVLA, enhanced ALMA sensitivity or large single-dish mm/submm telescopes, and wide-field FIR imaging spectroscopy.

References

Their physical origins—regulated by star-formation efficiency, feedback, and dark matter assembly—remain a major open question.

— Report of the Kavli-IAU Workshop on Global Coordination, "Probing the Universe from far-infrared to millimeter wavelengths: future facilities and their synergies"  (2409.07570 - Committee et al., 2024) in Section 5 (Environments and Ecosystems of Galaxies), p. 26

But this has not yet been conclusively shown because simulations of structure formation based on the correct initial conditions and with sufficiently high resolution are not yet available.

— The relevance of the cosmic microwave background for cosmology  (2609.10672 - Kroupa et al., 9 Sep 2026) in Section 4.4, “Further constraints on viable cosmological models”

The physical mechanisms driving such intense star formation remain uncertain. Both major mergers and secular cold gas accretion have been proposed, yet neither scenario easily reproduces SFRs exceeding a couple thousand yr${-1}$ without invoking extreme conditions or suppressing feedback entirely.

— An ALMA Band 7 survey of SDSS/Herschel quasars in Stripe 82: II. The nature of FIR-bright quasars  (2609.16951 - Hatziminaoglou et al., 15 Sep 2026) in Section 1, Introduction

Without deeper spectroscopy, the role of an AGN in establishing the spectral characteristics of GN-z11 remains unsettled.

— SPURS: Massive Stars, Dense Gas, and Ly$α$ Escape in GN-z11 at $z = 10.6$  (2608.12699 - Chen et al., 13 Aug 2026) in Section 1, Introduction

As for the SFE, the value of $f_{}$ for individual galaxies is unknown in (early) galaxy formation, though there is an approximate expected range of $f_{} \sim [0.001,0.1]$ that is commonly used (e.g., ), with $f_{*} = 0.015$ being the standard local theoretical value .

— A model to mini-quench early galaxies by balancing stellar feedback and gas accretion  (2609.19265 - Luberto et al., 16 Sep 2026) in Section 2.2, “A simple galaxy growth model”

How such apparently mature systems assembled and evolved so rapidly, however, remains an open question.

— REBELS-25: multi-phase morphology and kinematics at z = 7.31  (2608.25982 - Rowland et al., 26 Aug 2026) in Section 1, Introduction

However, the interpretation of this high-redshift galaxy tension remains open.

— Small-Scale Clustering of Primordial Black Holes: The Little Red Dot Mass Function and the High-Redshift Galaxy Tension  (2609.09078 - Zhang et al., 8 Sep 2026) in Section 4, “Accelerating early massive galaxy formation with SMBHs from PBH clusters”