The AURORA Survey: Robust Helium Abundances at High Redshift Reveal a Subpopulation of Helium-enhanced Galaxies in the Early Universe

Abstract We present the first robust helium (He) abundance measurements in star-forming galaxies at redshifts 1.6 ≲ z ≲ 3.3 using deep, moderate-resolution JWST/NIRSpec spectroscopy from the AURORA survey. We establish a high- z He sample consisting of 20 galaxies with multiple high-signal-to-noise-ratio (>5 σ ) He i emission-line detections, including the critical near-infrared λ 10833 line. This is the first study at high redshift leveraging λ 10833 to break degeneracies between temperature, electron density, optical depth, and He + /H + , enabling reliable He abundance determinations in the early Universe. We use a custom Markov Chain Monte Carlo framework incorporating direct-method electron temperature priors, extended optical depth ( τ λ 3890 ) model grids up to densities of 10 6 cm −3 , and simultaneous fits of the physical conditions and He i /H i line ratios to derive ionic He + /H + abundances. Most of the AURORA galaxies follow the extrapolated z ∼ 0 He/H–O/H trend, indicating modest He enrichment by z ∼ 2–3. However, we identify a subpopulation of four galaxies that exhibit elevated He mass fractions (Δ Y > 0.03) without corresponding enhancements in N/O or α -elements (∼20% of the sample). This abundance pattern is inconsistent with enrichment from asymptotic giant branch stars, but favors early He enrichment from very massive stars ( M ≳ 100 M ⊙ ), which can eject He-rich, N-poor material via stellar winds and binary stripping in young stellar populations. We speculate that these elevated-He systems may represent an early phase of globular cluster (GC) formation where N enrichment is still lagging behind He production. This work demonstrates the power of JWST multi-line He i spectroscopy for tracing early stellar feedback, enrichment pathways, and GC progenitor signatures in the high-redshift Universe.