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The ionizing fluxes from quasars and other active galactic nuclei (AGN) arecritical for interpreting their emission-line spectra and for photoionizing andheating the intergalactic medium (IGM). Using far-ultraviolet spectra from theCosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST), wedirectly measure the rest-frame ionizing continua and emission lines for 159AGN at redshifts 0.001 < z_AGN < 1.476 and construct a composite spectrum from475-1875A. We identify the underlying AGN continuum and strong EUV emissionlines from ions of oxygen, neon, and nitrogen after masking out absorptionlines from the HI Lya forest, 7 Lyman-limit systems (N_HI > 10^17.2 cm^-2) and214 partial Lyman-limit systems (15.0 < log N_HI < 17.2). The 159 AGN exhibit awide range of FUV/EUV spectral shapes, F_nu \propto nu^(alpha_nu), typicallywith -2 < alpha_nu < 0 and no discernible continuum edges at 912A (H I) or 504A(He I). The composite rest-frame continuum shows a gradual break at 1000 A,with mean spectral index alpha_nu = -0.83 +/- 0.09 in the FUV (1200-2000A)steepening to alpha_nu = -1.41 +/- 0.15 in the EUV (500-1000A). We discuss theimplications of the UV flux turnovers and lack of continuum edges for thestructure of accretion disks, AGN mass inflow rates, and luminosities relativeto Eddington values.

HST-COS Observations of AGN. II. Extended Survey of Ultraviolet Composite Spectra from 159 Active Galactic Nuclei