HST-COS OBSERVATIONS OF AGNs. I. ULTRAVIOLET COMPOSITE SPECTRA OF THE IONIZING CONTINUUM AND EMISSION LINES

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 (Ly alpha lambda 1216, C IV lambda 1549, Si IV/O IV] lambda 1400, N V lambda 1240, O VI lambda 1035). Strong EUV emission lines with 14-22 eV excitation energies (Ne VIII lambda lambda 770, 780, Ne V lambda 569, O II lambda 834, O III lambda 833, lambda 702, O IV lambda 788, 608, 554, O V lambda 630, N III lambda 685) suggest the presence of hot gas in the broad emission-line region. The rest-frame continuum, F-nu proportional to nu(alpha nu), shows a break at wavelengths lambda < 1000 angstrom, with spectral index alpha(nu) = -0.68 +/- 0.14 in the FUV (1200-2000 angstrom) steepening to alpha(nu) = -1.41 +/- 0.21 in the EUV (500-1000 angstrom). The COS EUV index is similar to that of radio-quiet AGNs in the 2002 HST/FOS survey (alpha(nu) = -1.57 +/- 0.17). We see no Lyman edge (tau(HI) < 0.03) or He I lambda 584 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 angstrom spectral break.