Electron Densities of Typical Low-Mass Galaxies at z ≃2–7 from Stacked JWST/NIRSpec Spectra

Direct electron-density measurements at high redshift are usually limited to galaxies with individually strong density-sensitive doublets, and therefore may not trace the average interstellar medium of ordinary low-mass galaxies. We stack public JWST/NIRSpec medium-resolution spectra from the DAWN JWST Archive to measure [SII]-based electron densities ne for low-mass galaxies at 2 < z < 7. The stacks yield ne ≃ 100–150 cm−3 at 2 < z < 5 and nₑ=381⁺¹⁰⁴₋₈₉\ cm⁻³ at 5 < z < 7, corresponding to an evolution ne = ne, 0[(1 + z)/(1 + 2.3)]α with ne,0=76⁺²²₋₂₃\ cm⁻³ and α =1.88+0.60-0.64. A mass-matched stacking test gives a consistent rising trend, indicating that the increase is not driven solely by changing stellar-mass distributions. Individually measurable galaxies with both [SII] components detected at S/N > 5 have a higher normalization, ne,0=211⁺³⁶₋₃₁\ cm⁻³, showing that individual-doublet samples select a denser subset. Stacking archival JWST spectra therefore provides a direct route to measuring the average gas density of low-mass galaxies below the individual-doublet detection threshold.