THE NATURE OF THE ULTRAVIOLET CONTINUUM IN TYPE-2 SEYFERT-GALAXIES

Type 2 Seyfert nuclei are well known to contain a ''featureless contin uum'' which makes a significant contribution in the optical and ultrav iolet. However, the nature of this featureless continuum is not clear. Recent optical spectropolarimetry shows that only a minor part of the optical featureless continuum can be light from a hidden Seyfert 1 nu cleus scattered into our line of sight. In this paper, we show that th is is also true in the ultraviolet. We have used International Ultravi olet Explorer (IUE) spectra of 20 of the brightest type 2 Seyfert nucl ei to construct an ultraviolet spectral template. While the continuum is well detected in the template, there is no detectable broad-line re gion (BLR). Comparing this template to a similar spectral template of type 1 Seyfert nuclei implies that no more than 20% of the Seyfert 2 t emplate's continuum can be light from a hidden Seyfert 1 nucleus scatt ered by dust or warm electrons. One obvious possibility is that most o f the nuclei in our sample are ''pure'' type 2 Seyfert galaxies that d o not contain a hidden type 1 Seyfert nucleus (e.g., we have a clear v iew of the central engine in the ultraviolet, and it simply lacks a br oad-line region). This is not compatible with the evidence that at lea st some Seyfert 2 galaxies can be unified with Seyfert 1 galaxies on t he basis of viewing geometry, unless there are two types of Seyfert 2 galaxies. As an alternative, we consider the possibility of Seyfert 1 light scattered off very hot electrons (T > 10(7) K). The BLR emission lines can then be broadened beyond recognition in our data. However, a scatterer this hot is inconsistent with optical spectropolarimetry. Optically-thin thermal emission from the type of warm mirror seen in N GC 1068 cannot produce the ultraviolet continuum we observe because th e equivalent width of Ly alpha and He II lambda 1640 are at least an o rder of magnitude too small in the Seyfert 2 template, and the observe d ultraviolet continuum is generally much too red to be thermal emissi on from gas warmer than 10(5) K. We discuss an alternative in which mo st of the ultraviolet continuum in these Seyfert 2 galaxies may be pro duced by a reddened starburst: a circumnuclear population of massive s tars which is unusually luminous in type 2 Seyfert galaxies compared t o normal galaxies of the same Hubble type. We show that our Seyfert 2 template is consistent with existing IUE spectra of metal-rich starbur sts in all salient properties. Two consequences of this would be that a significant fraction of the optical Balmer emission lines (as measur ed in an aperture as large as that of IUE) and most of the far-infrare d continuum detected from these Seyfert 2 galaxies would be powered by the starburst. Combining this inference with the evidence that many t ype 2 Seyfert nuclei contain a ''hidden'' type 1 Seyfert nucleus would then imply that both compact active nuclei and starbursts play import ant energetic roles in the Seyfert phenomenon. We also show that if NG C 1068 were at the median distance of the Seyfert 2 galaxies in our sa mple, then about 70% of its ultraviolet continuum as observed through the IUE aperture would arise in its circumnuclear star-forming ring. T hus, the ''anomalous'' behavior of the featureless continuum in NGC 10 68 compared to other Seyfert 2 galaxies may be caused at least in part by an aperture effect. Although the type 2 Seyfert template is not of adequate quality to allow the direct spectroscopic detection of massi ve stars, if such stars are present then this should be possible in th e near future with the Hubble Space Telescope (HST). Such observations will also be required to ascertain the relationship between the ultra violet continuum seen by IUE and the unpolarized component of the opti cal featureless continuum (the ''FC2''). In an appendix we consider wh ether any genuine ''type 2 quasars'' have yet been detected at high re dshift.