ON THE ORIGIN OF THE ULTRAVIOLET CONTINUUM IN SEYFERT-2 GALAXIES

High-resolution Hubble Space Telescope ultraviolet images of the Seyfe rt 2 galaxies NGC 1667, NGC 3982, NGC 5427, and NGC 5953, previously k nown to have circumnuclear star-forming rings, are presented. The UV i mages of all four Seyfert 2 galaxies show the presence of several comp act UV-bright knots at distances of similar to 0.7-2 kpc from the nucl eus and associated to the circumnuclear star-forming ring, The optical ly bright Seyfert 2 nucleus is barely detected in the UV. The UV image s provide the first direct empirical evidence that the UV flux emitted by galaxies having a Seyfert 2-type nucleus surrounded by a circumnuc lear star-forming ring is dominated by radiation coming from clusters of young hot stars distributed along the star-forming ring. The contri bution of the Seyfert 2 nucleus amounts to similar to 1%-10% of the ob served UV flux in these four galaxies. If circumnuclear star-forming r ings of 2-3 kpc in diameter, or smaller, are a common feature of Seyfe rt 2 galaxies, we conclude that (a) the observed UV Aux in Seyfert 2 g alaxies is not dominated by the radiation from the nuclear ionizing so urce and (b) the observed UV flux is dominated by emission from cluste rs of young hot stars located in circumnuclear star-forming regions. W hatever the nature of the nuclear ionizing source in these Seyfert 2 g alaxies, this must be obscured. The UV and Ha luminosities associated to the Seyfert 2 nucleus in these galaxies are consistent with the sce nario of the nuclear ionizing source being an obscured power-law ioniz ing source or a young (2-4 Myr) obscured high-metallicity (greater tha n or equal to Z(.)) nuclear starburst.