On the Origin of the Ultraviolet Continuum in Seyfert 2 Galaxies

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Abstract
High-resolution Hubble Space Telescope ultraviolet images of the Seyfert 2 galaxies NGC 1667, NGC 3982, NGC 5427, and NGC 5953, previously known to have circumnuclear star-forming rings, are presented. The UV images of all four Seyfert 2 galaxies show the presence of several compact UV-bright knots at distances of ~0.7-2 kpc from the nucleus and associated to the circumnuclear star-forming ring. The optically bright Seyfert 2 nucleus is barely detected in the UV. The UV images provide the first direct empirical evidence that the UV flux emitted by galaxies having a Seyfert 2-type nucleus surrounded by a circumnuclear star-forming ring is dominated by radiation coming from clusters of young hot stars distributed along the star-forming ring. The contribution of the Seyfert 2 nucleus amounts to ~1%-10% of the observed UV flux in these four galaxies. If circumnuclear star-forming rings of 2-3 kpc in diameter, or smaller, are a common feature of Seyfert 2 galaxies, we conclude that (a) the observed UV flux in Seyfert 2 galaxies is not dominated by the radiation from the nuclear ionizing source and (b) the observed UV flux is dominated by emission from clusters of young hot stars located in circumnuclear star-forming regions. Whatever the nature of the nuclear ionizing source in these Seyfert 2 galaxies, this must be obscured. The UV and Hα luminosities associated to the Seyfert 2 nucleus in these galaxies are consistent with the scenario of the nuclear ionizing source being an obscured power-law ionizing source or a young (2-4 Myr) obscured high-metallicity (≥Z☉) nuclear starburst.