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 $n_e$ for low-mass galaxies at $2<z<7$. The stacks yield $n_e\simeq 100$--$150\mathrm{c}{\mathrm{m}}^{-3}$ at $2<z<5$ and $n_e=381_{-89}^{+104}\mathrm{c}{\mathrm{m}}^{-3}$ at $5<z<7$, corresponding to an evolution $n_e=n_{e,0}[(1+z)/(1+2.3){]}^{\alpha }$ with $n_{e,0}=76_{-23}^{+22}\mathrm{c}{\mathrm{m}}^{-3}$ and $\alpha =1.88_{-0.64}^{+0.60}$. 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, $n_{e,0}=211_{-31}^{+36}\mathrm{c}{\mathrm{m}}^{-3}$, 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.

Accepted for publication in MNRAS