JWST/MIRI reveals the true number density of massive galaxies in the early Universe

Abstract

Early JWST studies reporting an unexpected abundance of massive galaxies at $z\sim 5$--$8$ challenge galaxy formation models in the $\Lambda$CDM framework. Previous stellar mass ($M_{\star }$) estimates suffered from large uncertainties due to the lack of rest-frame near-infrared data. Using deep JWST/NIRCam and MIRI photometry from PRIMER, we systematically analyze massive galaxies at $z\sim 3$--$8$, leveraging rest-frame $\gtrsim 1\mu$m constraints. We find MIRI is critical for robust $M_{\star }$ measurements for massive galaxies at $z>5$: excluding MIRI overestimates $M_{\star }$ by $\sim 0.4$ dex on average for $M_{\star }>10^{10}{M}_{\odot }$ galaxies, with no significant effects at lower masses. This reduces number densities of $M_{\star }>10^{10}{M}_{\odot }$ ($10^{10.3}{M}_{\odot }$) galaxies by $\sim 36\%$ ($55\%$). MIRI inclusion also reduces ``Little Red Dot'' (LRD) contamination in massive galaxy samples, lowering the LRD fraction from $\sim 32\%$ to $\sim 13\%$ at $M_{\star }>10^{10.3}{M}_{\odot }$. Assuming pure stellar origins, LRDs exhibit $M_{\star }\sim 10^{9\text{–}10.5}{M}_{\odot }$ with MIRI constraints, rarely exceeding $10^{10.5}{M}_{\odot }$. Within standard $\Lambda$CDM, our results indicate a moderate increase in the baryon-to-star conversion efficiency ($\epsilon$) toward higher redshifts and masses at $z>3$. For the most massive $z\sim 8$ galaxies, $\epsilon \sim 0.3$, compared to $\epsilon \lesssim 0.2$ 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.

published by ApJL, updated to match the published version

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