Beyond Extreme Burstiness: Evolving Star Formation Efficiency as the Key to Early Galaxy Abundance

TL;DR

This study uses a semi-empirical model with evolving star formation efficiency to reconcile JWST early galaxy observations with standard cosmology, avoiding the need for extreme burstiness.

Contribution

It introduces a physically motivated double power-law SFE model with redshift evolution in the low-mass slope, fitting observations across $z=4-16$ without extreme scatter.

Findings

Best-fit model matches observed luminosity functions and galaxy bias.

Redshift evolution in low-mass SFE slope is essential for fit.

Modest UV scatter (~0.32 dex) suffices for high-redshift galaxy data.

Abstract

JWST observations have revealed an overabundance of bright galaxies at $z \geq 9$, creating apparent tensions with theoretical predictions within standard $\Lambda$CDM cosmology. We address this challenge using a semi-empirical approach that connects dark matter halos to observed UV luminosity through physically motivated double power-law star formation efficiency (SFE) model as a function of halo mass, redshift and perform joint Bayesian analysis of luminosity functions spanning $z = 4 - 16$ using combined HST and JWST data. Through systematic model comparison using information criteria (AIC, BIC, DIC), we identify the optimal framework requiring redshift evolution only in the low-mass slope parameter $\alpha(z)$ while maintaining other SFE parameters constant. Our best-fitting model achieves excellent agreement with observations using modest, constant UV scatter $\sigma_{\rm UV} = 0.32$ dex, significantly lower than the $\gtrsim 1.3$ dex values suggested by previous studies for $z > 13$. This reduced scatter requirement is compensated by strongly evolving star formation efficiency, with $\alpha$ increasing toward higher redshifts, indicating enhanced star formation in low-mass halos during cosmic dawn. The model also successfully reproduces another important observational diagnostic such as effective galaxy bias and cosmic Star Formation Density (SFRD) consistently across the full redshift range. Furthermore, model predictions are consistent up to a redshift of $z\sim 20$. Our results demonstrate that JWST's early galaxy observations can be reconciled with standard cosmology through the interplay of modest stochasticity and evolving star formation physics, without invoking extreme burstiness or exotic mechanisms.