HST-COS Observations of AGNs. III. Spectral Constraints in the Lyman Continuum from Composite COS/G140L Data

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 Angstroms, 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 $\lambda_{\rm rest}\sim 850~{\rm Angstroms}$ down to $\lambda_{\rm rest}\sim 425~{\rm Angstroms}$. Parameterizing this slope as a power law, we obtain $F_\nu\propto \nu^{ -0.72\pm 0.26}$, 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 HeI 504 Angstrom photoelectric absorption edge opacity to $\tau_{\rm HeI}<0.047$.