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

TL;DR

Using JWST data, this study refines stellar mass estimates of early Universe galaxies, revealing lower number densities and reduced contamination, which impacts galaxy formation models within the b1bcdbcm framework.

Contribution

This work demonstrates the critical role of MIRI data in accurately measuring stellar masses of high-redshift galaxies, reducing overestimations and contamination in galaxy samples.

Findings

MIRI data reduces stellar mass overestimates by 0.4 dex for galaxies >10^{10}b0 M_7

Number densities of massive galaxies decrease by 36b0 to 55b0 with MIRI inclusion

High-redshift massive galaxies show increased baryon-to-star efficiency, up to b0.3 at z b0 8.

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.