Probing nuclear starburst activity in a sample of nearby spiral galaxies

Abstract

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 populations in spiral galaxies with radial velocity <= 5000 km/s. In this sample of 60 galaxies the IUE aperture probes a mean 1.0 kpc x 2.1 kpc area. Prior to any comparison of galaxy spectra in the range covered by IUE (1200-3200 AA), 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 have 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 signal-to-noise ratio templates have been obtained, and information on spectral 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 population syntheses using as base elements: ion {H}{ii} regions, integrated star clusters, and far-UV weak elliptical galaxies as representative of bulge stellar population. The variety of UV spectral types found in the central regions of spiral galaxies can be readily explained by different mixtures of bulge, circumnuclear burst and disc populations. Across different morphological types, similar templates can also be found. This is due to compensation effects of bulge contribution with the disc and circumnuclear burst ones. Flux fractions derived from the population synthesis have been converted into mass contributions and inferences have been made on the star-formation histories. In the central kpc of the galaxies with strong UV flux, we find that the mass stored in the young components (t<500 Myr) is typically \approx10^7 M_odot. We confirm that such star-formation enhancements occur preferentially in barred spirals. Internal reddening in the templates has been studied and inferences have been made on the corresponding reddening laws. We find cases where an SMC-like law applies and others where a faint lambda2200 AA absorption feature occurs resembling the reddening law of the LMC. The interest of the IUE data set resides in its rather large entrance aperture which samples a large portion of nearby galaxies, and is therefore quite suitable for the interpretation of large redshift galaxies. Based upon data collected with the International Ultraviolet Explorer (IUE) Satellite, supported by NASA, SERC and ESA.