Science with the Hubble Space Telescope -- II
Book Editors: P. Benvenuti, F. D. Macchetto, and E. J. Schreier
Electronic Editor: H. Payne
W. Zheng, G. A. Kriss, R. C. Telfer, J. P. Grimes & A. F. Davidsen
Center for Astrophysical Sciences, The Johns Hopkins University
Abstract:
We construct a composite quasar spectrum from 149 HST FOS spectra of 80
quasars with mean redshift 
. The spectrum covers the
wavelengths
between 600Å and 3000Å in the rest frame, with a peak S/N level of
per Å.
There is a significant steepening of the continuum slope around 1000Å.
The continuum longward of 1000Å can be modeled as a power law 
with 
. Between 600Å and
1000Å the power--law index 
.
The break in the power--law index is a feature expected in Comptonized
accretion--disk spectra. Electron scattering in a
corona above the disk produces a power--law tail in the
wavelength band shortward of 
Å.
The spectral features in the sub--Ly
region of AGN spectra provide
critical insights into
the physical processes around the central engine, and they become accessible
to IUE and HST at intermediate redshift. O'Brien, Gondhalekar, & Wilson
(1988) analyzed the IUE data and found a gradual
steepening of the quasar continuum from the near--infrared to far--UV band.
The Hubble Space Telescope (HST), with its superb light--collecting ability,
has greatly extended our probe of the distant universe. The HST Faint
Object Spectrograph (FOS) database is superior
to the IUE database in terms of S/N level and spectral resolution.
During the last five years, a
significant number of FOS spectra of quasars have been accumulated. As part of
an archival study, we have constructed a composite spectrum of quasars based
on 149 FOS spectra of 80 quasars. It reveals some spectral features which are
not noticed in individual spectra, particularly in the far--UV range.
Archival FOS spectra are retrieved from the Space Telescope Science
Institute in the form of calibrated data.
The target selection criteria are: (1) redshift 
;
(2) no broad absorption line is present. Our sample includes
41 spectra with grating G160L, 5 with G130H, 40 with G190H, 49 with G270H,
and 14 with G400H. The G160L and G130H spectra are checked for proper
background subtraction and are reprocessed if necessary.
The FOS spectra are corrected for Galactic extinction, and then shifted to
the quasar rest frame.
Data points in the window near the significant airglow lines at 1216Å and
1304Å are replaced by values linearly interpolated from neighboring
parts of the spectrum. If a spectrum contains an obvious flux
discontinuity due to an external Lyman--limit system,
only the undepressed portion of the spectrum is used.
Each spectrum is resampled into 0.1Å bins, and then normalized to
match the average flux of a template spectrum which covers the whole spectral
region. The initial template is generated by adding all the spectra with equal
weights. The spectra are subsequently combined pixel
by pixel, weighted by 
where f is the flux and e the
error. A combination process with this weighting is similar to count
summation of raw data. Only the data points with average 
are used. The resultant spectrum serves
as the new template spectrum, and the normalization and combining processes
are repeated to reduce discontinuities
until a smooth final spectrum is produced. Figure 1 displays the number of
merged spectra vs. wavelength.
The composite quasar spectrum is shown in Figure 2.
The spectral region near 1200Å has the
highest S/N level, 
per 0.1Å, and the region near 700Å has
.
The flux depression shortward of the Ly
emission due to Ly
forest--line
absorption is small, at a level not more than 10%.
The continuum can be approximated with
a broken power law. The power--law index (
) is
longward of
1000Å and 
at shorter wavelengths.
The break point of the power law appears to be near 1000Å.
The change in the continuum shape around 1000Å is so
significant and abrupt that it cannot be explained by reddening or
intervening absorption, commonly known as the Lyman Valley
(Møller & Jakobsen 1990).
The continuum shape in the merged spectrum matches the characteristics of a
Comptonized spectrum of an accretion disk with Lyman--limit absorption.
The thermal emission from
a geometrically thin, optically thick accretion disk around a central
black hole, plus an underlying power law, may well fit the continuum
shape longward of 1000Å (Sun & Malkan 1989). The continuum at
shorter wavelengths may be modified by electron scattering in a corona
above the disk (Czerny & Zbyszewska 1991).
Comptonization produces a power--law high--energy tail and
smears out any Lyman--limit discontinuity feature intrinsic to the disk.
Such a feature is significant in the spectra of several quasars, but not
detected in most objects.
A number of emission lines are present. In addition to the major emission
features marked in Figure 2, some weak features, such as C III 
977
and N III 
991, can be measured because of the
high S/N level. The emission feature around 690Å may be O III+N
III arising from Bowen fluorescence (Eastman & MacAlpine 1985).
The presence of far--UV emission lines beyond O VI suggests a
high temperature (
K) in the region emitting these
lines as their excitation energies are greater than 12 eV.
References:
Czerny, B. & Zbyszewska, M. 1991, MNRAS, 249, 643
Eastman, R. G. & MacAlpine, G. M. 1985, ApJ, 299, 785
Møller, P. & Jakobsen, P. 1990, A&A, 228, 299
O'Brien, P. T., Gondhalekar, P. M., & Wilson, R. 1988, MNRAS, 233, 801
Sun, W. & Malkan, M. A. 1989, ApJ, 346, 68
payne@stsci.edu