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J/ApJ/475/469                 Composite HST Spectrum of Quasars (Zheng+ 1997)
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A Composite HST Spectrum of Quasars
     Zheng W., Kriss G.A., Telfer R.C., Grimes J.P., Davidsen A.F.
    <Astrophys. J. 475, 469 (1997)>
    =1997ApJ...475..469Z
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ADC_Keywords: QSOs ; Redshifts ; Photometry, ultraviolet
Keywords: atomic processes - quasars: general - ultraviolet: galaxies
Mission_Name: HST

Abstract:
    We construct a composite quasar spectrum from 284 HST FOS spectra of
    101 quasars with redshifts z>0.33. The spectrum covers the wavelengths
    between 350 and 3000A in the rest frame, with a peak S/N level of
    ~130 per A at ~1200A. Since ~90% of the sample quasars have redshift
    z<1.5, the spectrum is suitable for studying the wavelength region
    shortward of Ly{alpha} without large effects from intervening
    Ly{alpha} forest absorption. Data in the waveband between 350 and 600A
    are mainly from the spectra of z>1.5 quasars, for which significant
    corrections for the accumulated Lyman-series line and continuum
    absorption have been applied. There is a significant steepening of the
    continuum slope around 1050A. The continuum between 1050 and 2200A can
    be modeled as a power law f_{nu} proportional to {nu}_{alpha}_ with
    {alpha}=-0.99+/-0.05. For the full sample the power-law index in the
    extreme ultraviolet (EUV) between 350 and 1050A is
    {alpha}=-1.96+/-0.15. For the radio-loud subsample (60 objects), the
    EUV spectral index is {alpha}~=-2.2, while for the radio-quiet
    subsample (41 objects) it is {alpha}~=-1.8. The continuum flux in the
    wavelengths near the Lyman limit shows a depression of ~10%. The break
    in the power-law index and the slight depression of the continuum near
    the Lyman limit are features expected in Comptonized accretion-disk
    spectra. Comptonization produces a power-law tail in the wavelength
    band shortward of 1000A and smears out the Lyman-limit edge of the
    intrinsic accretion-disk spectrum. In the EUV waveband, we detect
    several possible emission features, including one around 690A that may
    be O III + N III produced by the Bowen fluorescence effect. Comparing
    our composite spectrum with one made at higher redshifts by Francis et
    al. (1991ApJ...373..465F), we find that the equivalent widths of
    Ly{alpha} and high-ionization emission lines are larger in our sample,
    reflecting a known luminosity dependence. The equivalent widths of
    low-ionization lines do not exhibit such a dependence, suggesting that
    the quasar EUV continuum above ~50eV is steeper at higher luminosity.
    Radio-quiet quasars appear to show a slightly harder continuum and
    lower ionization levels in their emission lines.

Description:
  The table data contain the composite spectra from figures 5 and 8 in
  the printed paper. The table captions are:

     FIG. 5. Composite FOS spectrum of 101 quasars, binned to 2A.
     Prominent emission lines and the Lyman limit are labeled, and two
     possible emission features are marked. The continuum fitting
     windows are marked with the bars near the bottom.

     FIG. 8. Composite FOS spectra of 60 radio-loud and 41 radio-quiet
     quasars, binned to 2A. The flux level in the spectrum for
     radio-quiet quasars is shifted down for display purposes.

File Summary:
--------------------------------------------------------------------------------
 FileName     Lrecl      Records       Explanations
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ReadMe           80            .       This file
fig5.dat         34        10249       Composite quasar spectrum
fig8a.dat        34        10187       Composite spectrum of radio-loud quasars
fig8b.dat        34         8307       Composite spectrum of radio-quiet quasars
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Byte-by-byte Description of file: fig5.dat fig8a.dat fig8b.dat
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   Bytes Format  Units       Label      Explanations
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   1- 8   F8.3   0.1nm       Lambda     Wavelength
  11-18   F8.5   ---         Flux       Relative F(lambda)
  20-26   F7.4   ---       e_Flux       Error in Flux
  30-34   F5.3   ---         RMS        RMS deviation per merging spectrum as
                                         a fraction of flux
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Origin: AAS CD-ROM series, Volume 8, 1997        Lee E. Brotzman [ADS] 06-May-97
Note: Files in the CD-Rom (directory /volume8.apj/v475/p469) are in error.
      We thank Dr Zheng <zheng@hut4.pha.jhu.edu>
       for providing the correct files.
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(End)                                                         [CDS]  04-Jul-1997