J/ApJ/475/469 Composite HST Spectrum of Quasars (Zheng+ 1997)
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 1997ApJ...475..469Z
ADC_Keywords: QSOs ; Redshifts ; Photometry, ultraviolet
Mission_Name: HST
Keywords: atomic processes - quasars: general - ultraviolet: galaxies
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 Lyα without large effects from intervening
Lyα 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_ν proportional to να with
α=-0.99±0.05. For the full sample the power-law index in the
extreme ultraviolet (EUV) between 350 and 1050A is
α=-1.96±0.15. For the radio-loud subsample (60 objects), the
EUV spectral index is α~=-2.2, while for the radio-quiet
subsample (41 objects) it is α~=-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 1991ApJ...373..465F), we find that the equivalent widths of
Lyα 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:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
fig5 34 10249 Composite quasar spectrum
fig8a 34 10187 Composite spectrum of radio-loud quasars
fig8b 34 8307 Composite spectrum of radio-quiet quasars
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Byte-by-byte Description of file: fig5 fig8a fig8b
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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
for providing the correct files.
(End) [CDS] 04-Jul-1997