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For refcode 1998ApJ...509...93W:
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1998ApJ...509...93W Toward a Unified Model for the "Diffuse Ionized Medium" in Normal and Starburst Galaxies JING WANG, TIMOTHY M. HECKMAN, AND MATTHEW D. LEHNERT Received 1998 April 13; accepted 1998 July 17 ABSTRACT 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] {lambda}{lambda}6716, 6731. In this paper we analyze H{alpha} 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 H{alpha} surface brightness ({SIGMA}_H{alpha}_) and the [S II]/H{alpha} 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{alpha} 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{alpha} surface brightness ({SIGMA}_H{alpha}+/{SIGMA}_e_, where {SIGMA}_e_ is the mean H{alpha} 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 ({SIGMA}_SFR_). Good quantitative agreement with the data follows if we require the constant of proportionality to be consistent with the values of P and {SIGMA}_SFR_ in the local disk of the Milky Way. Such a scaling between P and {SIGMA}_SFR_ may arise either because feedback from massive stars heats the ISM or because {SIGMA}_SFR_ is determined (or limited) by the mean gas pressure. Subject headings: galaxies: ISM-galaxies: starburst-ISM: kinematics and dynamics-ISM: structure
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