1998ApJ...509...93W

The ``diffuse ionized medium'' (DIM) makes up a significant fraction of the mass and ionization requirements of the interstellar medium of the Milky Way and is now known to be an energetically significant component in most normal star-forming galaxies. Observations of the ionized gas in starburst galaxies have revealed the presence of gas with striking similarities to the DIM in normal galaxies: relatively low surface brightness and strong emission from low-ionization forbidden lines like [S II] λλ6716, 6731. In this paper we analyze Hα images and long-slit spectra of samples of normal and starburst galaxies to better understand the nature of this diffuse, low surface brightness gas. We find that in both samples there is a strong inverse correlation between the Hα surface brightness (Σ_Hα) and the [S II]/Hα line ratio at a given location in the galaxy. However, the correlation for the starbursts is offset brightward by an order of magnitude in Hα 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 Hα surface brightness (Σ_Hα/Σ_e, where Σ_e is the mean Hα 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.