The true number density of massive galaxies in the early Universe revealed by JWST/MIRI

One of the main challenges in galaxy formation that has emerged recently is the early assembly of massive galaxies. The observed number density and the maximum stellar mass (M_⋆) of massive galaxies in the early Universe appear to be higher than model predictions, which may pose a serious problem to the LCDM cosmology. A major limitation in many previous studies is the large uncertainty in estimating M_⋆ due to the lack of constraints in the rest-frame near-infrared part of the spectral energy distribution, which is critical to determining M_⋆ accurately. Here we use data from a large JWST/MIRI survey in the PRIMER program to carry out a systematic analysis of massive galaxies at z ∼ 3-8, leveraging photometric constraints at rest-frame ≳ 1 μm. We find a significant reduction in the number and mass densities of massive galaxies at z > 5 compared to earlier results that did not use the MIRI photometry. Within the standard ΛCDM cosmology, our results require a moderate increase in the baryon-to-star conversion efficiency (ϵ) towards higher redshifts and higher M_⋆. For the most massive galaxies at z∼ 8, the required ϵ is ∼ 0.3, in comparison to ϵ∼ 0.14 for typical low-redshift galaxies. Our findings are consistent with models assuming suppressed stellar feedback due to the high gas density and the associated short free-fall time expected for massive halos at high redshift.