Ultraviolet spectral properties of magellanic and non-magellanic irregulars, H II and Starburst galaxies

This paper presents the results of a stellar population analysis performed on nearby (VR ≤ 5 000 kms-ˡ) starforming galaxies, comprising magellanic and non-magellanic irregulars, H II and starburst galaxies observed with the IUE satellite. Before any comparison of galaxy spectra, we have formed subsets according to absolute magnitude and morphological classification. Subsequently, we have coadded the spectra within each subset into groups of similar spectral properties in the UV. As a consequence, high signal-to-noise ratio templates have been obtained, and information on spectral features can now be extracted and analysed. Seven groups resulted from this procedure: the magellanic irregulars (including H II galaxies) produced two different blue spectral groups; the nonmagellanic irregulars could be grouped into two spectral groups with rather peculiar properties; and the luminous starbursts produced one flat and two blue template spectra. Their stellar populations are analysed by means of a population synthesis algorithm based on star cluster spectral components. The synthetic spectra reproduce the observed ones successfully (except the non-magellanic irregular groups) both in terms of continuum distribution and spectral features. The synthesis flux fractions of different age groups were transformed into mass fractions, allowing inferences on the star formation histories. Young stellar populations (age < 500 Myrs) are the main flux contributors; in a few cases the intermediate age population (age ≈ 1–2 Myrs) is important, while the old bulge population contributes at most with ≈ 2%of the λ2646 Å flux in the case of starburst galaxies, and is negligible in the magellanic irregulars.We also study the reddening values and the extinction law: an SMC-like extinction law is appropriate for all cases