| Summary: | The rapid decline in the average star formation activity of galaxies from z~2.6 to z~0 is accompanied by an order of magnitude decrease in the typical electron densities of HII regions. However, it is unclear what connects the pc-scale properties of the line-emitting gas with the global properties of the host galaxies. I will summarize unique new results from a study of 611 galaxies at 0 < z < 2.6, drawn primarily from the KMOS^3D and SAMI surveys. We measure both the local electron density of the lineemitting gas and the volume-averaged electron density across the starforming disks at four different redshifts, yielding unique constraints on the volume filling factor and evolving spatial distribution of ionized gas in HII regions. We compare our measurements to quantitative predictions to evaluate whether the electron density evolution is most likely to be driven by changes in the strength of stellar feedback, the molecular cloud density, and/or the hydrostatic equilibrium pressure.
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