EMISSION-LINE RATIOS OF THE INTEGRATED SPECTRA OF GALAXIES - EVIDENCEFOR A DIFFUSE IONIZED MEDIUM IN OTHER GALAXIES

We have put the optical emission-line ratios of the integrated spectra of nearby galaxies-as recently measured by Kennicutt-on the standard line-ratio diagrams used to classify extragalactic emission-line objec ts. We restrict our analysis to galaxies with Halpha emission-line equ ivalent widths > 30 angstrom (in order to avoid systematic errors in t he measured line ratios caused by underlying stellar absorption lines) . This subsample consists mostly of normal star-forming galaxies of mi ddle to late Hubble type. We then find that the line ratios in the int egrated spectra are intermediate between those of individual high surf ace-brightness H II regions and those of active galactic nuclei. Speci fically, for a given ratio of [O III]/Hbeta, the integrated spectra ha ve ratios of [S II]/Halpha that are typically approximately 20%-100% h igher than those seen in H II regions. The integrated spectra have rat ios of [N II]/Halpha that are on-average only approximately 0.1 dex la rger than those seen in H II regions. However, in some galaxies the en hancement in [N II]/Halpha is up to a factor of 2 over those seen in H II regions. These results imply that the integrated emission-line spe ctra of star-forming galaxies have a substantial contribution from gas with significantly different characteristics than those of the high s urface-brightness gas associated with H II regions. The enhanced relat ive strength of the low ionization emission-lines is qualitatively sim ilar to what is observed for the diffuse ionized gas in our own and so me nearby spiral and irregular galaxies. Assuming that the line ratios in the diffuse ionized gas in the Milky Way and NGC 891 are typical, we find that at least 25% of the observed Halpha flux from an average star-forming galaxy would have to arise in this diffuse component. Thi s material is probably either shock-ionized by supernovae and stellar winds or photoionized by diffuse starlight. In either case, this has i nteresting implications for the ionization and heating of the interste llar medium.