Beyond No Tension: JWST z > 10 Galaxies Push Simulations to the Limit

TL;DR

The study tests whether JWST-detected galaxies at $z>10$ challenge the Renaissance simulations under $\Lambda$CDM by comparing MoM-z14 and GS-z14 to the RP/Normal regions and evaluating the $z=15$ UV luminosity function. MoM-z14's mass is compatible with RP predictions, while GS-z14 remains an outlier, signaling possible gaps in high-mass, high-redshift star formation modeling. UV luminosity functions from observations tend to lie below the brightest simulated systems, suggesting refinements in star formation prescriptions or resolution rather than a fundamental cosmological conflict. Overall, the results indicate no strong tension with $\Lambda$CDM, but highlight the need for environment-specific adjustments in early-universe galaxy formation models and caution regarding sample variance in JWST observations.

Abstract

JWST has identified some of the Universe's earliest galaxies, repeatedly pushing the frontier to ever higher redshifts and stellar masses. The presence of such extreme galaxies at such early times, with large stellar populations and high star-formation rates, naturally results in a tension between observation and theory. This tension between numerical models and observations can be either due to our underlying cosmological models or due to a gap in our understanding of early Universe astrophysics. In a prelude to this letter, we showed how the Renaissance simulations, which focused on high redshift galaxy formation were able to reconstruct similar stellar masses to the earliest and highest mass galaxies that had been discovered by JWST at the time of its publication (McCaffrey et al.2023). Since then many more galaxies have been discovered by JWST, in particular the "Mirage-or-Miracle" (MoM) survey broke the record recently with the highest redshift galaxy MoM-z14, which has a spectroscopically confirmed redshift of $z \sim 14.44$ followed closely by GS-z14 with a spectroscopically confirmed redshift of $z \sim 14.3$. We investigate in this letter whether these newly discovered galaxies are in conflict with the Renaissance simulations and thus whether they are causing tension with our established models of cosmology and/or high-redshift astrophysics. We discover that MoM-z14's high mass at early redshift can be explained by the Renaissance simulation suite, whereas the extremely high stellar mass of GS-z14 remains an outlier when compared to previous measurements of high-redshift galaxies detected by JWST and our numerical models (even after accounting for cosmic variance).

Figures (2)

• Figure 1: The growth evolution of the most massive galaxies from each region in Renaissance compared with observations from JWST. The dashed lines indicate where the stellar mass is extrapolated after the end time of each region in Renaissance, using a specific star formation rate of $10^{-8}$. The shaded regions denote the upper limit to these galaxies if we take into account an error from cosmic variance of $+100\%$. The greyscale lines denoted by $dn/dz$ indicate the probability of discovering a galaxy of a certain stellar mass and at a certain redshift in the FoV of the NIRCam instrument on JWST. The calculation of these lines can be found in Appendix A of mccaffreyNoTensionJWST2023c.
• Figure 2: Luminosity function of the galaxies modelled in Renaissance, along with the luminosity function derived from the parameters constrained from GS-z14 and MoM-z14 naiduCosmicMiracleRemarkably2025. We have also included estimates of the luminosity function derived from models which assume a top-heavy IMF in the early Universe (brown line) and a model which assumes that star formation efficiencies increase with increasing redshift mauerhoferSynergisingSemianalyticalModels2025. It is noted that the luminosity function derived by naiduCosmicMiracleRemarkably2025 assumes that these galaxies are the only $-21 < M_{\rm UV} < -20$ galaxies to exist at $z\sim14-15$ in the UDS, GOODS-S and COSMOS fields, imaged by the PRIMER and JOF surveys.