2025ApJ...993L..51L

The interstellar medium in high-redshift galaxies exhibits significantly higher electron densities (n_e) than in the local Universe. To investigate the origin of this trend, we analyze a sample of 9590 centrally star-forming galaxies with stellar masses greater than 10⁹M_⊙ at redshifts 0.01 < z < 0.04, selected from the Dark Energy Spectroscopic Instrument Data Release 1. We derive n_e from the [S II] λλ6716, 6731 doublet, measuring values of n_e = 30-400 cm^–3 at z ≈ 0. We find a tight correlation between n_e and the star formation rate surface density (Σ_SFR), which is well described by a broken power law. Above a threshold of $\mathrm{log}\,({{\rm{\Sigma }}}_{\,\rm{SFR}\,}/{M}_{\odot }\,{\,\rm{yr}\,}^{-1}\,{\,\rm{kpc}\,}^{-2})\geqslant -1.46$, the relation follows $\rm{}{n}_{{\rm{e}}}\rm{}=(233\pm 13)\times {{\rm{\Sigma }}}_{\,\rm{SFR}\,}^{0.49\pm 0.02}$. Below this threshold, n_e remains approximately at a constant value of 44 ± 3 cm^–3. Remarkably, this relation remains consistent with measurements of galaxies at z = 0.9-10.2. By converting the observed redshift evolution of Σ_SFR into n_e evolution through our n_e-Σ_SFR relation, we obtain n_e = 40 × (1 + z)^1.4 cm^–3, consistent with previous direct observations. The n_e-Σ_SFR relation arises likely because the high Σ_SFR, fueled by dense cold gas or elevated efficiency, enhances radiative and mechanical feedback and produces dense ionized gas whose n_e are further regulated by ambient pressure. We conclude that the redshift evolution of n_e primarily reflects the evolution of the cold gas density and star formation activity over cosmic time.