Star formation in distant starburst galaxies

Abstract

This paper discusses the stellar population content of distant (5 000kms−1 VR 16 000kms−1) galaxies with enhanced star-formation activity. Distinction is made between isolated galaxies and galaxies morphologically disturbed, with clear signs of interaction such as mergers. In these galaxies the International Ultraviolet Explorer (IUE) large aperture samples most of the galaxy’s body. Consequently, the resulting integrated spectra arise primarily from blue stellar populations of different ages together with significant contributions from intermediate and old age components, subject to varying reddening amounts. Instead of analysing individual, usually lowSignal-to- Noise ratio (S/N) spectra, our approach is to coadd the spectra of objects with similar spectral properties in the UV, considering as well their properties in the visible/near-infrared ranges. Consequently, the resulting high (S/N) template spectra contain the average properties of a rather uniform class of objects, and information on spectral features can now be analysed with more precision. Three groups have been found for the interacting galaxies, corresponding to a red, blue and very blue continuum. Isolated galaxies have been separated into two groups, one with a flat/red continuum and the other with a blue continuum. For comparison, we also include in the present analysis two groups of nearby disturbed galaxies. Stellar populations are analysed by means of a synthesis algorithm based on star cluster spectral components of different ages which fit the observed spectra both in terms of continuum distribution and spectral features. Flux fractions of the different age groups found in the synthesis have been transformed into mass fractions, allowing inferences on the star formation histories. Young stellar populations (age < 500 Myr) are the main flux contributors, except for the groups with a red spectrum not due to extinction, arising from the intermediate (age 1−2 Gyr) and old age populations.We also study the reddening values and the extinction law: a Small Magellanic Cloud-like extinction law is appropriate for all cases. As compared to nearby galaxies with enhanced star-formation, the distant starburst galaxy spectral groups exhibit larger contributions from the intermediate and old age populations. This effect is mainly accounted for by the larger spatial area sampled by the IUE slit in the distant galaxies, including not only the entire bulge but also evolved disk populations. The present results provide a quantitative measure of the star-forming activity in interacting galaxies, compared to isolated galaxies.