[Sii]λ6716/[Sii]λ6731 line flux ratio as a function of electron density (cm^-3), for a temperature of 10⁴ K (blue line). Only results for the stacks of the brightest pixels of Hα emission are indicated (red crosses). The error bars indicate the 1σ uncertainties in the measurements. For clarity, stacks with [Sii] line ratios below the low-density limit are not shown. The inset at the lower left shows the distribution of electron densities n_e in the nuclear region of nearby starburst galaxies (Lehnert & Heckman 1996), as well as the mean electron density for the stack, including only the brightest pixels of all our objects (solid blue line) and the corresponding  ± 1σ uncertainties (dashed blue lines).