HE 0515–4414: an unusual sub-damped Ly α system revisited ^*,**,***

¹ Brockmann Consult, GKSS-Forschungszentrum / GITZ, Max-Planck-Straße 2, 21502 Geesthacht, Germany e-mail: ralf.quast@brockmann-consult.de
² Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany e-mail: [dreimers;rbaade]@hs.uni-hamburg.de

Received: 24 December 2005
Accepted: 8 October 2007

Abstract

Aims.We examine the ionization, abundances, and differential dust depletion of metals, the kinematic structure, and the physical conditions in the molecular hydrogen-bearing sub-damped Ly α system toward HE 0515-4414.

Methods.We used the STIS and VLT UVES spectrographs to obtain high-resolution recordings of the damped Ly α profile and numerous associated metal lines. Observed element abundances were corrected with respect to dust depletion effects.

Results.The sub-damped Ly α absorber at redshift is unusual in several aspects. The velocity interval of associated metal lines extends for 700 km s^-1. In addition, saturated H I absorption is detected in the blue damping wing of the = 8  10¹⁹ cm^-2 main component. The column density ratios of associated Al II, Al III, and Fe II lines indicate that the absorbing material is ionized. Nineteen of a total of 31 detected metal line components are formed within peripheral H II regions, while only 12 components are associated with the predominantly neutral main absorber. The bimodal velocity distribution of metal line components suggests two interacting absorbers. For the main absorber, the observed abundance ratios of refractory elements to Zn range from a Galactic warm disk [Si/Zn]_g = -0.40 ± 0.06, [Fe/Zn]_g = -1.10 ± 0.05 to halo-like and essentially undepleted patterns. The dust-corrected metal abundances indicate a nucleosynthetic odd-even effect and might imply an anomalous depletion of Si relative to Fe for two components, but otherwise they do correspond to solar ratios. The intrinsic average metallicity is almost solar [Fe/H]_m = -0.08 ± 0.19, whereas the uncorrected average is [Zn/H]_g = -0.38 ± 0.04. The ion abundances in the periphery conform to solar element composition.

Conclusions.The detection of H II, as well as the large variation in dust depletion for this sight line, raises the question whether in future studies of damped Ly α systems ionization and depletion effects have to be considered in more detail. Ionization effects, for instance, may mimic an enrichment of α elements. An empirical recipe for detecting H II regions is provided.