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

Abstract

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 $n_e=381^{+104}_{-89}\ {\rm cm^{-3}}$ at 5 < z < 7, corresponding to an evolution ne = ne, 0[(1 + z)/(1 + 2.3)]α with $n_{e,0}=76^{+22}_{-23}\ {\rm cm^{-3}}$ and $\alpha =1.88^{+0.60}_{-0.64}$. 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, $n_{e,0}=211^{+36}_{-31}\ {\rm cm^{-3}}$, 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.