2012ApJ...752..162S

The ionizing fluxes from quasars and other active galactic nuclei (AGNs) are critical for interpreting the emission-line spectra of AGNs 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 AGNs. Over the redshift range 0.026 < z < 1.44, COS samples the Lyman continuum and many far-UV emission lines (Lyα λ1216, C IV λ1549, Si IV/O IV] λ1400, N V λ1240, O VI λ1035). Strong EUV emission lines with 14-22 eV excitation energies (Ne VIII λλ770, 780, Ne V λ569, O II λ834, O III λ833, λ702, O IV λ788, 608, 554, O V λ630, N III λ685) suggest the presence of hot gas in the broad emission-line region. The rest-frame continuum, F_ν ∝ν^αν_^, shows a break at wavelengths λ < 1000 Å, with spectral index α_ν = -0.68±0.14 in the FUV (1200-2000 Å) steepening to α_ν= -1.41 ±0.21 in the EUV (500-1000 Å). The COS EUV index is similar to that of radio-quiet AGNs in the 2002 HST/FOS survey (α_ν= -1.57±0.17). We see no Lyman edge (τ_ H I_< 0.03) or He I λ584 emission in the AGN composite. Our 22 AGNs exhibit a substantial range of FUV/EUV spectral indices and a correlation with AGN luminosity and redshift, likely due to observing below the 1000 Å spectral break.