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The ionizing fluxes from quasars and other active galactic nuclei (AGN) arecritical for interpreting the emission-line spectra of AGN and forphotoionization and heating of the intergalactic medium. Using ultravioletspectra from the Cosmic Origins Spectrograph (COS) on the Hubble SpaceTelescope (HST), we have directly measured the rest-frame ionizing continua andemission lines for 22 AGN. Over the redshift range 0.026 < z < 1.44, COSsamples the Lyman continuum and many far-UV emission lines (Lya 1216, C IV1549, Si IV/OIV] 1400, N V 1240, O VI 1035). Strong EUV emission lines with14-22 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 inthe 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 inthe EUV (500-1000 A). The COS EUV index is similar to that of radio-quiet AGNin 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 asubstantial range of FUV/EUV spectral indices and a correlation with AGNluminosity and redshift, likely due to observing below the 1000 A break.

HST-COS Observations of AGN. I. Ultraviolet Composite Spectra of the Ionizing Continuum and Emission Lines