  The ionizing fluxes from quasars and other active galactic nuclei (AGN) are
critical for interpreting their emission-line spectra and for photoionizing and
heating the intergalactic medium (IGM). Using far-ultraviolet spectra from the
Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST), we
directly measure the rest-frame ionizing continua and emission lines for 159
AGN at redshifts 0.001 < z_AGN < 1.476 and construct a composite spectrum from
475-1875A. We identify the underlying AGN continuum and strong EUV emission
lines from ions of oxygen, neon, and nitrogen after masking out absorption
lines from the HI Lya forest, 7 Lyman-limit systems (N_HI > 10^17.2 cm^-2) and
214 partial Lyman-limit systems (15.0 < log N_HI < 17.2). The 159 AGN exhibit a
wide range of FUV/EUV spectral shapes, F_nu \propto nu^(alpha_nu), typically
with -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 the
implications of the UV flux turnovers and lack of continuum edges for the
structure of accretion disks, AGN mass inflow rates, and luminosities relative
to Eddington values.
