JWST/MIRI Reveals the True Number Density of Massive Galaxies in the Early Universe

Early JWST studies reporting an unexpected abundance of massive galaxies at z ∼ 5-8 challenge galaxy formation models in the ΛCDM framework. Previous stellar mass (M_⋆) estimates suffered from large uncertainties due to the lack of rest-frame near-infrared data. Using deep JWST/NIRCam and Mid-Infrared Instrument (MIRI) photometry from Public Release IMaging for Extragalactic Research, we systematically analyze massive galaxies at z ∼ 3-8, leveraging rest-frame ≳1 μm constraints. We find MIRI is critical for robust M_⋆ measurements for massive galaxies at z > 5: excluding MIRI overestimates M_⋆ by ∼0.4 dex on average for M_⋆ > 10¹⁰M_⊙ galaxies, with no significant effects at lower masses. This reduces number densities of M_⋆ > 10¹⁰M_⊙ (10^10.3M_⊙) galaxies by ∼36% (55%). MIRI inclusion also reduces “Little Red Dot” (LRD) contamination in massive galaxy samples, lowering the LRD fraction from ∼32% to ∼13% at M_⋆ > 10^10.3M_⊙. Assuming pure stellar origins, LRDs exhibit M_⋆ ∼ 10^9-10.5M_⊙ with MIRI constraints, rarely exceeding 10^10.5M_⊙. Within standard ΛCDM, our results indicate a moderate increase in the baryon-to-star conversion efficiency (ϵ) toward higher redshifts and masses at z > 3. For the most massive z ∼ 8 galaxies, ϵ ∼ 0.3, compared to ϵ ≲ 0.2 for typical galaxies at z < 3. This result is consistent with models where high gas densities and short free-fall times suppress stellar feedback in massive high-z halos.