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Дата изменения: Wed Jun 15 19:38:45 2005 Дата индексирования: Sat Dec 22 11:24:11 2007 Кодировка: Поисковые слова: redshift survey |
S. Köhler, A. de la Varga and D. Reimers
Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
) QSO HE1122-1649 (Reimers et al. 1995a,1995b).
The combination of HST and optical spectra will offer the possibility to
study heavy-element abundances in a damped Ly
absorber at low
redshift. Observations of such low-redshift absorbers are essential in
order to study the evolution of the metal and dust content in damped
Ly
absorbers which are likely the progenitors of present day disk
galaxies (Wolfe 1993).
Keywords: absorption lines
Ultraviolet data have been obtained quite recently with the Faint Object Spectrograph onboard the HST. With gratings G270H and G190H the wavelength range from 1600 to 3280Å has been covered at a resolution of R=1300 with a maximum signal-to-noise ratio of 28.
Optical data have been taken with CASPEC at the 3.6m telescope
at ESO/La Silla. Forty three echelle orders cover the wavelength range from
3750 to 5250Å at a resolution of R=15500 and a signal-to-noise ratio
> 40 longward of the Ly
emission line and > 25 in the Ly
forest. The combined data are shown in Figure 1.
The combination of HST and optical spectra will offer
the possibility
to study heavy-element abundances in a damped Ly
absorber at low
redshift.
Observations of such low-redshift absorbers are essential in order to
study the evolution of the metal and dust content in damped Ly
absorbers.
Surprisingly,
the lack of chemical evolution as derived from observations of a
damped Ly
absorber at 
towards 3C286 as well as a damped Ly
absorber at 
towards
PKS0454+039 hints at a slowly evolving galaxy population
(Meyer & York 1992, Steidel et al. 1995).
The optical data of HE1122-1649 show a strong low-ionization absorption
system (MgII, FeII) at 
which appears to be
responsible
for the Lyman limit at 
1550Å seen in IUE data.
In addition to the strong Ly
absorption, we also detect the higher
Lyman series lines Ly
and Ly
related to this system in the HST
data.
Calculating Voigt profiles
for the Lyman series lines yields a neutral hydrogen column density
of about logN(HI)
20.3 cm
.
Apparently, the
Ly
absorption line is blended with several absorption lines, which
hinders the fitting of the line wings by a Voigt
profile. The Ly
profile reveals an absorbing subcomponent at
in agreement with results from heavy element absorption lines
detected in the high resolution optical data.
Figure:
Combined optical and ultraviolet spectra
of the z=2.4 Quasar HE1122-1649.
Strongest absorption lines of the damped Ly
absorbing system at 
are indicated.
Since numerous heavy element absorption lines of different strength are observed,
it is possible to identify at least
5 subcomponents at 
, 0.6820, 0.6822, 0.6825, 0.6829.
Figure 2 illustrates profiles for 10 resonance lines detected in the optical data
which have been transformed to Doppler velocities where v=0 corresponds
to a vacuum heliocentric redshift of 
.
The FeII absorption features show broad asymmetric profiles
with strong absorption between v=-110 and +50 kms
. Such
line profile asymmetries observed for heavy element absorption lines
in damped Ly
systems at redshifts 
were explained
by absorption in a rotating thick, metal poor gaseous disk (Wolfe et
al. 1994). Next to the highest column density components we observe
weaker discrete MgII absorber systems at velocities v=-217
kms
and v=+64 kms
. The observed decrease in the
strength of MgII systems at low redshift might indicate less
clustering of these systems (Bergeron et al. 1994, Steidel &
Sargent 1992). However, our detection confirms the presence of
strong MgII absorbers with numerous subcomponents at low
redshifts.
In the HST data we observe strong absorption at the expected positions
of CII1334, SiII1190, 1193, 1260, 1527,
SiIII1206, AlII1670
related to the 
absorbing complex. Weak
absorption is visible for SiIV1393, 1402 and CIV1548,
1550.
However, in the Ly
forest one is always faced with a blending problem.
From optical data further heavy-element absorbing systems toward
HE1122-1649 can be easily identified. We find two weak
low-ionization systems (MgII, FeII) at z=0.8062 and
. Ly
, Ly
, Ly
, Ly
lines
related to these absorbers are detected in the HST data, which will
allow an accurate estimate of the neutral hydrogen column density. In
addition, we find strong absorption at the expected positions of
CIV1548, 1550, SiII1260 and SiIII1206.
We also detect a high-ionization system (CIV) at 
with Lyman series lines up to HI923.15 visible in the HST
data. This system shows i.a. absorption by OIII702,
OIV554, 553, 787, OV629 and CIII977.
Due to the high absorption line density shortward of the Ly
emission line
a quantitative analysis has to await
the identification
of all heavy-element absorbing systems as well as
Ly
clouds in
order to disentangle blends.
This work has been supported by the Verbundforschung of the Bundesministerium für Forschung und Technologie under No. 50 OR 90058.
Bergeron, J., Petitjean, P., Sargent, W. L. W. et al. 1994, ApJ, 436, 33
Reimers, D., Rodríguez-Pascual, P., Hagen, H.-J., & Wisotzki, L. 1995, A&A, 293, L21
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Figure: Velocity profiles of heavy element absorption lines arising in the
damped Ly
system as seen in the optical data.
The velocity v=0 kms
corresponds to a vacuum heliocentric
redshift 
. The optical data
have been normalized to unit continuum.