NASA/IPAC EXTRAGALACTIC DATABASE
Date and Time of the Query: 2019-08-18 T04:39:30 PDT
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For refcode 2014ApJ...794...75S:
Retrieve 159 NED objects in this reference.
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NED Abstract

Copyright by American Astronomical Society. Reproduced by permission
2014ApJ...794...75S HST-COS Observations of AGNs. II. Extended Survey of Ultraviolet Composite Spectra from 159 Active Galactic Nuclei Stevans, Matthew L.; Shull, J. Michael; Danforth, Charles W.; Tilton, Evan M. Abstract. 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_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_H\,{\scriptsize I} >= 10^{17.2} cm^-2^) and 214 partial Lyman-limit systems (14.5 < log N_H\,{\scriptsize I}< 17.2). The 159 AGNs exhibit a wide range of FUV/EUV spectral shapes, F_{{nu}}_ is proportional to {nu} ^{{alpha}_{{nu}}_}, typically with -2 <= {alpha}_nu_ <= 0 and no discernible continuum edges at 912 A (H I) or 504 A (He I). The composite rest-frame continuum shows a gradual break at lambda_br_ ~ 1000 A, with mean spectral index {alpha}_nu_ = -0.83 +/- 0.09 in the FUV (1200-2000 A) steepening to {alpha}_nu_ = -1.41 +/- 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. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS5-26555. Key words: galaxies: active, line: profiles, quasars: emission lines, ultraviolet: galaxies
Retrieve 159 NED objects in this reference.
Please click here for ADS abstract

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