S. Köhler, A. de la Varga and D. Reimers
Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
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.
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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.