Determine the dominant physical mechanism behind the compactness–density relation

Determine whether the observed correlation between galaxy-scale stellar surface density and average nebular electron density is driven primarily by gas confinement, the concentration of star formation, or aperture-integrated mixing of classical H II regions and diffuse ionized gas.

Background

The paper reports a population-level correlation between kpc-scale stellar compactness, quantified by stellar surface density, and the average electron density inferred from stacked [S II] emission-line spectra. After matching stellar mass and star-formation rate, the authors discuss several possible physical explanations: a deeper stellar potential and higher mid-plane pressure could confine denser star-forming clouds; compact galaxies could have higher star-formation-rate surface densities and different distributions of ionizing sources; or compactness could alter the relative contributions of classical H II regions and diffuse ionized gas within the spectroscopic aperture. Because the spectra are unresolved, the analysis does not determine which mechanism dominates. The authors identify spatially resolved spectroscopy combining [S II] with independent pressure, ionization-parameter, gas-surface-density, diffuse-gas, and resolved star-formation-density diagnostics as necessary to resolve this question.