PROBING NUCLEAR STARBURST ACTIVITY IN A SAMPLE OF NEARBY SPIRAL GALAXIES

As part of a systematic study of the UV properties of galaxies in the IUE library, we present in this paper an analysis of nuclear stellar p opulations in spiral galaxies with radial velocity less than or equal to 5 000 km s(-1). In this sample of 60 galaxies the IUE aperture prob es a mean 1.0 kpc x 2.1 kpc area. Prior to any comparison of galaxy sp ectra in the range covered by IUE (1200-3200 Angstrom), we have formed subsets according to the absolute magnitude and morphological type of the studied galaxies. In a second step, and within each subset, we ha ve co-added the spectra, and hence the objects into groups of similar spectral properties in the UV, also taking into account their spectral properties in the visible/near-infrared ranges. As a result, high sig nal-to-noise ratio templates have been obtained, and information on sp ectral features can now be extracted and interpreted. We distinguish 4 groups for Sa, 8 for Sb, and 4 for Sc galaxies. We have carried out p opulation syntheses using as base elements: H II regions, integrated s tar clusters, and far-UV weak elliptical galaxies as representative of bulge stellar population. The variety of UV spectral types found in t he central regions of spiral galaxies can be readily explained by diff erent mixtures of bulge, circumnuclear burst and disc populations. Acr oss different morphological types, similar templates can also be found . This is due to compensation effects of bulge contribution with the d isc and circumnuclear burst ones. Flux fractions derived from the popu lation synthesis have been converted into mass contributions and infer ences have been made on the star-formation histories. In the central k pc of the galaxies with strong UV flux, we find that the mass stored i n the young components (t < 500 Myr) is typically approximate to 10(7) M.. We confirm that such star-formation enhancements occur preferenti ally in barred spirals. Internal reddening in the templates has been s tudied and inferences have been made on the corresponding reddening la ws. We find cases where an SMC-like law applies and others where a fai nt lambda 2200 Angstrom absorption feature occurs resembling the redde ning law of the LMC. The interest of the IUE data set resides in its r ather large entrance aperture which samples a large portion of nearby galaxies, and is therefore quite suitable for the interpretation of la rge redshift galaxies.