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