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Galaxies in the early Universe appear to have grown too big too fast, assembling into massive, monolithic objects more rapidly than anticipated in the hierarchical Lambda cold dark matter (ΛCDM) structure formation paradigm. The available photometric data are consistent with there being a population of massive galaxies that form early (z ≳ 10) and quench rapidly over a short (≲1 Gyr) timescale, consistent with the traditional picture for the evolution of giant elliptical galaxies. Similarly, kinematic observations as a function of redshift show that massive spirals and their scaling relations were in place at early times. Explaining the early emergence of massive galaxies requires either an extremely efficient conversion of baryons into stars at z > 10 or a more rapid assembly of baryons than anticipated in ΛCDM. The latter possibility was explicitly predicted in advance by modified Newtonian dynamics (MOND). We discuss some further predictions of MOND, such as the early emergence of clusters of galaxies and early reionization. [ABSTRACT FROM AUTHOR]

Recent observations of high-redshift galaxies ( z ≲ 7) reveal that a substantial fraction have turbulent, gas-rich disks with well-ordered rotation and elevated levels of star formation. In some instances, disks show evidence of spiral arms, with bar-like structures. These remarkable observations have encouraged us to explore a new class of dynamically self-consistent models using our agama / Ramses hydrodynamic N -body simulation framework that mimic a plausible progenitor of the Milky Way at high redshift. We explore disk gas fractions of f _gas = 0%, 20%, 40%, 60%, 80%, and 100% and track the creation of stars and metals. The high gas surface densities encourage vigorous star formation, which in turn couples with the gas to drive turbulence. We explore three distinct histories: (i) there is no ongoing accretion and the gas is used up by the star formation, (ii) the star-forming gas is replenished by cooling in the hot halo gas, and (iii) in a companion paper, we revisit these models in the presence of a strong perturbing force. At low f _disk (≲0.3), where f _disk is the baryon mass fraction of the disk relative to dark matter within 2.2 R _disk , a bar does not form in a stellar disk; this remains true even when gas dominates the inner disk potential. For a dominant baryon disk ( f _disk ≳ 0.5) at all gas fractions, the turbulent gas forms a strong radial shear flow that leads to an intermittent star-forming bar within about 500 Myr; turbulent gas speeds up the formation of bars compared to gas-free models. For f _gas ≲ 60%, all bars survive, but for higher gas fractions, the bar devolves into a central bulge after 1 Gyr. The star-forming bars are reminiscent of recent discoveries in high-redshift Atacama Large Millimeter/submillimeter Array observations of gaseous disks.

Recent photometric detections of extreme ( z > 10) redshift galaxies from the JWST have been shown to be in strong tension with existing simulation models for galaxy formation and in the most acute case, in tension with ΛCDM itself. These results, however, all rest on the confirmation of these distances by spectroscopy. Recently, the JADES survey has detected the most distant galaxies with spectroscopically confirmed redshifts, with four galaxies found with redshifts between z = 10.38 and z = 13.2. In this Letter, we compare simulation predictions from four large cosmological volumes and two zoom-in protoclusters with the JADES observations to determine whether these spectroscopically confirmed galaxy detections are in tension with existing models for galaxy formation or with ΛCDM more broadly. We find that existing models for cosmological galaxy formation can generally reproduce the observations for JADES in terms of galaxy stellar masses, star formation rates, and the number density of galaxies at z > 10.