HST-COS Observations of AGN. I. Ultraviolet Composite Spectra of the
  Ionizing Continuum and Emission Lines
  The ionizing fluxes from quasars and other active galactic nuclei (AGN) are
critical for interpreting the emission-line spectra of AGN and for
photoionization and heating of the intergalactic medium. Using ultraviolet
spectra from the Cosmic Origins Spectrograph (COS) on the Hubble Space
Telescope (HST), we have directly measured the rest-frame ionizing continua and
emission lines for 22 AGN. Over the redshift range 0.026 < z < 1.44, COS
samples the Lyman continuum and many far-UV emission lines (Lya 1216, C IV
1549, Si IV/OIV] 1400, N V 1240, O VI 1035). Strong EUV emission lines with
14-22 eV excitation energies (Ne VIII 770,780, Ne V 569, O II 834, O III 833,
702, O IV 788,608,554, O V 630, N III 685) suggest the presence of hot gas in
the broad emission-line region. The rest-frame continuum, F_nu ~ nu^{alpha_nu},
shows a break at wavelengths below 1000 A, with spectral index alpha_nu = -0.68
+/- 0.14 in the FUV (1200-2000 A) steepening to alpha_nu = -1.41 +/- 0.21 in
the EUV (500-1000 A). The COS EUV index is similar to that of radio-quiet AGN
in the 2002 HST/FOS survey (alpha_nu = -1.57 +/- 0.17). We see no Lyman edge
(tau_HI < 0.03) or He I 584 emission in the AGN composite. Our 22 AGN exhibit a
substantial range of FUV/EUV spectral indices and a correlation with AGN
luminosity and redshift, likely due to observing below the 1000 A break.
