Astronomy and Astrophysics – Astrophysics
Scientific paper
May 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994apj...426l..27l&link_type=abstract
Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X), vol. 426, no. 1, p. L27-L30
Astronomy and Astrophysics
Astrophysics
38
Emission Spectra, H Alpha Line, H Beta Line, H Ii Regions, Interstellar Matter, Nitrogen, Oxygen Atoms, Star Formation, Stellar Composition, Stellar Spectrophotometry, Sulfur, Gaseous Diffusion, Interstellar Gas, Ionized Gases, Stellar Winds, Supernovae
Scientific paper
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 objects. We restrict our analysis to galaxies with H-alpha emission-line equivalent widths greater than 30 A (in order to avoid systematic errors in the measured line ratios caused by underlying stellar absorption lines). This subsample consists mostly of normal star-forming galaxies of middle to late Hubble type. We then find that the line ratios in the integrated spectra are intermediate between those of individual high surface-brightness H II regions and those of active galactic nuclei. Specifically, for a given ratio of (O III)/H-beta, the integrated spectra have ratios of (S II)/H-alpha that are typically approximately 20%-100% higher than those seen in H II regions. The integrated spectra have ratios of (N II)/H-alpha that are on-average only approximately 0.1 dex larger than those seen in H II regions. However, in some galaxies the enhancement in (N II)/H-alpha is up to a factor of 2 over those seen in H II regions. These results imply that the integrated emission-line spectra of star-forming galaxies have a substantial contribution from gas with significantly different characteristics than those of the high surface-brightness gas associated with H II regions. The enhanced relative strength of the low ionization emission-lines is qualitatively similar to what is observed for the diffuse ionized gas in our own and some 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 H-alpha flux from an average star-forming galaxy would have to arise in this diffuse component. This material is probably either shock-ionized by supernovae and stellar winds or photoionized by diffuse starlight. In either case, this has interesting implications for the ionization and heating of the interstellar medium.
Heckman Timothy M.
Lehnert Matthew D.
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