On nature of the 'diffuse ionized medium' in star-forming galaxies: An optical observational investigation

Details On nature of the 'diffuse ionized medium' in star-forming galaxies: An optical observational investigation On nature of the 'diffuse ionized medium' in star-forming galaxies: An optical observational investigation

Sep 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999phdt.........3w&link_type=abstract

Thesis (PhD). THE JOHNS HOPKINS UNIVERSITY, Source DAI-B 60/03, p. 1127, Sep 1999, 175 pages.

Physics

2

Scientific paper

We have carried out an optical observational program to study the global structure, physical state, and dynamics of the `Diffuse Ionized Medium' (`DIM') in a sample of the nearest and brightest late-type galaxies. We show that the structural properties of the DIM can be reasonably unified as a function of relative surface brightness (i.e. absolute surface brightness scaled by a mean Ha surface brightness). The DIM can be defined as having a small relative surface brightness (rather than an absolute value), The DIM and the giant HII regions differ systematically in their physical and dynamical state. The DIM shows enhanced emission in the low-ionization lines, and in the high-ionization [OIII] line as well. The DIM is also more disturbed kinematically than the HII region gas. The high-ionization gas in the DIM is more kinematically disturbed than the low-ionization gas. The differing kinematics implies that the DIM has a quiescent component (dominating Hα emission) with a low ionization state and a disturbed component with a high ionization state. We argue that the quiescent DIM is most likely photoionized and confined within the disk, and the disturbed DIM is most likely mechanically heated and more extended vertically (scale height of 0.5-1 kpc). We find no clear discontinuity between the properties of the HII regions and the quiescent DIM. We have also compared the DIM in our galaxies to the faint ionized gas in a starburst sample. We find that both samples define a universal inverse-correlation relation between [SII]/Hα line ratio and the relative Hα surface brightness. Such a universal correlation can be a natural outcome of a photoionization model in which the DIM characteristic thermal pressure is proportional to the mean star- formation rate per unit area. Good quantitative agreement with the data follows if the constant of proportionality is consistent with the local Galactic disk values of pressure and star-formation rate. We discuss possible causes for such a proportionality relation. The similar physical properties of the faint ionized gas in these galaxies suggest that the DIM is ubiquitous in all star- forming galaxies.

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