Gas properties of HII and starburst galaxies: relation with the stellar population

We study the gas emission of galaxies with active star formation, consistin g mostly of H II and starburst galaxies, as well as some Seyfert 2 galaxies , and determine chemical and physical parameters, The data consist of 19 hi gh signal-to-noise ratio optical templates, a result of grouping 185 emissi on-line galaxy spectra. Underlying stellar population models (from Raimann et al.) were subtracted from the templates in order to isolate the pure emi ssion component. We analyse the distribution of these improved signal-to-noise ratio emissio n spectra in diagnostic diagrams and find that the H II templates show a sm aller spread in log([O III]/H beta) values than the individual galaxies, ap parently as a result of the population subtraction and a better signal-to-n oise ratio. We thus suggest the template sequence as a fiducial observation al locus for H II galaxies which can be used as reference for models. The s equence of line ratios presented by the H II galaxies in the diagram log([O III]lambda 5007/H beta) versus log([N II]lambda 6584/H alpha) is primarily owing to the gas metallicity, of which the log([N II]/H alpha) ratio is a direct estimator. We also study the properties of the starburst galaxies an d those intermediate between H II and starburst galaxies, which are more me tal rich and sit on more massive galaxies. We discuss the present results in the frame of a recently proposed equivale nt-width diagnostic diagram for emission-line galaxies (by Rola et al.) and conclude that the observed ranges in W([O II])/W(H beta) and W(H beta) are mostly owing to the non-ionizing stellar population contribution. We propo se that W(H beta) be used as an estimator of this contribution to the conti nuum, and briefly discuss implications to the cosmological use of H II gala xies.