The rest-frame ultraviolet (UV) spectra of active galactic nuclei (AGNs) are important diagnostics of both accretion disk physics and their contribution to the metagalactic ionizing UV background. Though the mean AGN spectrum is well characterized with composite spectra at wavelengths greater than 912 ?, the shorter-wavelength extreme-UV (EUV) remains poorly studied. In this third paper in a series on the spectra of AGNs, we combine 11 new spectra taken with the Cosmic Origins Spectrograph on the Hubble Space Telescope with archival spectra to characterize the typical EUV spectral slope of AGNs from λrest?~?850 ? down to λrest?~?425 ?. Parameterizing this slope as a power law, we obtain Fν?∝?ν?0.72±0.26, but we also discuss the limitations and systematic uncertainties of this model. We identify broad emission features in this spectral region, including emission due to ions of O, Ne, Mg, and other species, and we limit the intrinsic He i 504 ? photoelectric absorption edge opacity to τHe i??0.047.
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机译：活跃银河原子核（AGN）的其余帧紫外线（UV）光谱是吸积盘物理学及其对偏银离子电离UV背景的贡献的重要诊断。尽管平均AGN光谱可以很好地表征波长大于912？的复合光谱，但对短波长的极紫外（EUV）仍然缺乏研究。在有关AGN光谱的系列文章的第三篇中，我们将用哈勃太空望远镜的宇宙起源光谱仪拍摄的11个新光谱与档案光谱相结合，以表征λrest？〜850？AGNs的典型AUV的EUV光谱斜率。下降到λrest？〜？425？。将这个斜率参数化为幂定律，可以获得Fν？∝？ν？0.72±0.26，但我们也讨论了该模型的局限性和系统不确定性。我们确定了该光谱区域中的宽发射特征，包括由于O，Ne，Mg和其他物种的离子引起的发射，并且限制了本征He i 504？光电吸收边缘不透明度为τHe i≤0.047。
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