First Statistical Detection of Mg ii -traced Cool Gas Outflows with JWST toward Cosmic Dawn

Abstract Galactic-scale outflows are a crucial component of galaxy evolution, yet their properties in the early Universe remain poorly constrained. We present the first statistical investigation of Mg ii -traced cool gas outflows in galaxies spanning a wide cosmic timeline from z ≈ 1 to z > 6 (with sample coverage extending to z ∼ 10). Using thousands of public JWST/NIRSpec spectra, we employ a signal-to-noise-weighted spectral stacking technique on the Mg ii λλ 2796, 2803 absorption doublet. We robustly detect blueshifted Mg ii absorption in nearly all stellar mass and redshift bins, with the exception of the lowest-mass systems at z ∼ 1–2. The outflow equivalent width exhibits a positive correlation with stellar mass ( M * ) at all epochs, with the fitted slope of 1.21 ± 0.35. Our work provides the first statistical constraints on Mg ii -traced cool outflows in the low-mass ( M * ≲ 10 9.5 M ⊙ ), high-redshift ( z > 3) regime. We also find that the outflow velocities generally remain below the host halo escape velocities, consistent with a galactic fountain scenario. The consistency of the stellar mass outflow equivalent width relation across z ∼ 2–6 suggests a persistent, unevolving feedback mechanism governing the baryon cycle toward cosmic dawn.