HST -COS OBSERVATIONS OF AGNs. II. EXTENDED SURVEY OF ULTRAVIOLET COMPOSITE SPECTRA FROM 159 ACTIVE GALACTIC NUCLEI

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...

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Bibliographic Details
Published in:The Astrophysical journal 2014-10, Vol.794 (1), p.1-17
Main Authors: Stevans, Matthew L, Shull, J Michael, Danforth, Charles W, Tilton, Evan M
Format: Article
Language:English
Subjects:
ABSORPTION
ACCRETION DISKS
Active galactic nuclei
ASTROPHYSICS, COSMOLOGY AND ASTRONOMY
Continuums
DATA
EMISSION
EXTREME ULTRAVIOLET RADIATION
FAR ULTRAVIOLET RADIATION
Fluxes
GALAXIES
GALAXY NUCLEI
HEATING
Hubble Space Telescope
IONS
LUMINOSITY
MASS
NEON
NITROGEN
OXYGEN
QUASARS
RED SHIFT
SPACE
SPECTRA
Spectral emissivity
TELESCOPES
Ultraviolet
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Summary: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 < z sub(AGN) < 1.476 and construct a composite spectrum from 475 to 1875 A. 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 Ly alpha forest, 7 Lyman-limit systems (N sub(H I) [> or =, slant] 10 super(17.2) cm super(-2)) and 214 partial Lyman-limit systems (14.5 < log N sub(H I) < 17.2). The 159 AGNs exhibit a wide range of FUV/EUV spectral shapes, F sub(v) [is proportional to] v super( alpha ) sub(v), typically with -2 [< or =, slant] alpha sub(v) [< or =, slant] 0 and no discernible continuum edges at 912A (H I) or 504 A (He I). The composite rest-frame continuum shows a gradual break at [lambda] sub(br) approximately 1000 A, with mean spectral index alpha sub(v) = -0.83 + or - 0.09 in the FUV (1200-2000 A) steepening to alpha sub(v) = -1.41 + or - 0.15 in the EUV (500-1000 A). 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.
ISSN:1538-4357
0004-637X
1538-4357
DOI:10.1088/0004-637X/794/1/75