Astronomy and Astrophysics, volume 342, 395-407 (1999/2-2)
A complex Lyman limit system at z=1.9 towards HS1103+6416.
KOEHLER S., REIMERS D., TYTLER D., HAGEN H.-J., BARLOW T. and BURLES S.
Abstract (from CDS):
We analyse absorption lines in optical and ultraviolet spectra of the bright (V=15.8, z=2.19) QSO HS1103+6416. High-resolution (FWHM=8km/s) optical spectra have been obtained with the Keck 10m telescope in the range from 3180 to 5780Å. Ultraviolet observations in the range from 1150 to 3280Å were performed with the FOS and the GHRS onboard the Hubble Space Telescope (HST). In this paper we concentrate our discussion on a complex Lyman limit system (LLS) at z=1.89. Absorption lines by carbon, silicon and aluminum in the optical spectra reveal a complex velocity structure with at least 11 components spanning a velocity range of 200km/s. From the Lyman limit in the ultraviolet spectra we derive a total neutral hydrogen column density of logN(HI)=17.46cm–2. Column densities of heavy elements in the individual components were derived by Voigt profile fitting. The eleven components can be subdivided roughly into three groups: Components 2, 3 and 6 with radial velocities v = -129... -95km/s with low ionization (L), components 4, 5, 7, 8 (v=-75... +2) with intermediate ionization (I), and components 1, 9, 10, 11 (v=-129, +34... +57) with high ionization (H). In order to study the ionization and abundances in these systems we compare the observed column densities with photoionization models. The observed absorption in the optical data can be explained by individual clouds with slightly varying metal abundances photoionized by slightly different radiation fields. Highly ionized components favour the extragalactic radiation field as calculated by Haardt & Madau (1996) while the components of low and intermediate ionization are better reproduced with a harder ionizing radiation field. Obviously local sources like stars can therefore be excluded as the main ionizing sources. Abundances in components L and I appear to be slightly different from those in the high ionization component H. In L and I we find roughly [C/H]=-0.9 while H has [C/H]=-1.2, consistent with the expectation that in a galaxy or groups of galaxies the abundances in the higher ionized `Halo' component are lower. The relative element abundances are also different. While in components L and I [Si/C]≃0.2, barely significant, and [S/C] and [O/C]≃0 within the uncertainties, component H shows [Si/C] = 0.5 and in addition [O/C] and [S/C] = 0.4 (both from HST spectra). [Al/C] measurable only in L and I is always ≃ 0. The tendency of enhanced α element (O, Si, S) abundances at low C abundance is consistent with what is known from nucleosynthesis theory (SNII dominant at the beginning of galactic evolution), from metal deficient stars in our galaxy and from QSO absorption line systems. If all components were ionized by the same radiation field the relative overabundances of O and S in the highly ionized components would be even larger. We show that HS1103+6416 will offer in the future for the first time the possibility to measure the cosmic He abundance at high redshift. Detailed calculations of Hei absorption using the multicomponent model which explains the metal lines shows consistency with the observed first seven series members of the HeI584, 537, 522Å ... series for a helium abundance Y=0.24, the expected cosmic He abundance from Big Bang nucleosynthesis modified by stellar nucleosynthesis at ∼1/10 solar metallicity. The presence of Oi and possibly Ovi absorption cannot be explained by our photoionization models and might hint at the existence of additional mainly neutral components with relatively low HI column density and further ionization mechanisms like, e.g., collisional ionization.