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Towards a Unified Model for the `Diffuse Ionized Medium' in Normal and Starburst Galaxies

We analyze H$α$ images and long-slit spectra of samples of normal and starburst galaxies to better understand the nature of the diffuse, low-surface-brightness gas in these galaxies. We find that in both samples there is a strong inverse correlation between the H$α$ surface-brightness ($Σ_{Hα}$) and the [SII]/H$α$ line ratio at a given location in the galaxy. However, the correlation for the starbursts is offset brightward by an order-of-magnitude in H$α$ surface-brightness at a given line ratio. In contrast, we find that all the galaxies (starburst and normal alike) define a universal relation between line ratio and the relative H$α$ surface brightness ($Σ_{Hα}/Σ_e$, where $Σ_e$ is the mean H$α$ surface brightness within the galaxy half-light radius). We show that such a universal correlation is a natural outcome of a model in which the DIM is photoionized gas that has a characteristic thermal pressure ($P$) that is proportional to the mean rate of star-formation per unit area in the galaxy ($Σ_{SFR}$). Good quantitative agreement with the data follows if we require the constant of proportionality to be consistent with the values of $P$ and $Σ_{SFR}$ in the local disk of the Milky Way. Such a scaling between $P$ and $Σ_{SFR}$ may arise either because feedback from massive stars heats the ISM or because $Σ_{SFR}$ is determined (or limited) by the mean gas pressure.