HST-COS observations of AGNs. II. Extended survey of ultraviolet composite spectra from 159 active galactic nuclei.
STEVANS M.L., SHULL J.M., DANFORTH C.W. and TILTON E.M.
Abstract (from CDS):
The ionizing fluxes from quasars and other active galactic nuclei (AGNs) are critical for interpreting their emission-line spectra and for photoionizing and heating the intergalactic medium. Using far-ultraviolet (FUV) 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 AGNs at redshifts 0.001 < zAGN< 1.476 and construct a composite spectrum from 475 to 1875 Å. We identify the underlying AGN continuum and strong extreme ultraviolet (EUV) emission lines from ions of oxygen, neon, and nitrogen after masking out absorption lines from the H I Lyα forest, 7 Lyman-limit systems (NHI ≥ 1017.2/cm2) and 214 partial Lyman-limit systems (14.5 < logNHI < 17.2). The 159 AGNs exhibit a wide range of FUV/EUV spectral shapes, Fv ∝ ναν^, typically with -2 ≤ αν ≤ 0 and no discernible continuum edges at 912 Å (H I) or 504 Å (He I). The composite rest-frame continuum shows a gradual break at λbr~ 1000 Å, with mean spectral index αν= -0.83±0.09 in the FUV (1200-2000 Å) steepening to αν= -1.41±0.15 in the EUV (500-1000 Å). 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.