Ultraviolet properties of early-type galaxies

Abstract

We analyse the UV properties of early-type galaxies from their UV spectra in the IUE library, including both normal and active nuclei. We co-added the spectra, and hence the objects into groups of similar spectral properties in the UV, also taking into account their properties in the visible/near-infrared ranges. Although, owing to the presence of a residual xed pattern noise, IUE data cannot be improved by co-addition as expected for spectra containing only random noise, this procedure still provided spectra of higher signal/noise ratio than in previous studies, often based on individual spectra and therefrom derived colour indices. Thanks to the co-adding procedure, information on spectral features can now be assessed. The red stellar population groups exhibit a far-UV flux at dierent levels. The higher ones denote the presence of the UV turnup, which possibly exhibits absorption features, at least as detected through the IUE aperture. As to the blue stellar population groups and/or AGNs, we have carried out simple syntheses with UV star cluster templates and galaxy spectra in order to infer the properties such as burst ages and contamination by an active nucleus. It has been possible to derive information on the extinction law aecting some internally reddened galaxies; there are cases where the presence of the 2200 A absorption feature suggests a reddening law similar to the Galactic one, and others without the 2200 Af eature, suggesting that it is rather an SMC type law which applies, then.