Early Bright Galaxies from Helium Enhancements in High-Redshift Star Clusters

TL;DR

This paper suggests that helium enhancements in high-redshift star clusters can significantly increase UV luminosity and stellar temperature, potentially explaining observed excesses in early galaxies and reducing model-observation tensions.

Contribution

It introduces stellar evolution models showing helium enrichment effects on UV brightness and temperature in high-redshift galaxies, a novel explanation for JWST observations.

Findings

Helium enhancement can boost UV luminosity by up to 50%.

Helium-rich stars have higher effective temperatures.

Helium effects may reconcile JWST data with galaxy formation models.

Abstract

The first few cycles of JWST have identified an overabundance of UV-bright galaxies and a general excess of UV luminosity density at $z\gtrsim10$ compared to expectations from most (pre-JWST) theoretical models. Moreover, some of the brightest high-redshift spectroscopically confirmed galaxies exhibit peculiar chemical abundance patterns, most notably extremely high N/O ratios. Since N/O has been empirically shown to scale strongly with He/H, as expected for hot hydrogen burning, these same bright high-redshift galaxies are likely also helium-enhanced. Under simplistic assumptions for stellar evolution, the bolometric luminosity of a star scales as $L\propto(2-\frac{5}{4}Y)^{-4}(2-Y)^{-1}$ -- hence a higher He/H leads to brighter stars. In this Letter, we evolve a series of MESA models to the zero-age main-sequence and highlight that the helium enhancements at the levels measured and inferred for high-redshift galaxies can boost the 1500 $\mathring{\rm A}$ UV luminosity by up to $\sim50\%$, while simultaneously increasing the stellar effective temperature. The combination of helium enhancements with nebular continuum emission expected for intense bursts of star formation have the potential to help reduce the tension between JWST observations and certain galaxy formation models.