The sub-damped Ly alpha system toward HE 0515-4414
Creators: name: Quast, Ralf
Publisher: name: Zenodo
Year: 2006
Subjects: damped Lyman-alpha systems
evolution strategies
fine-structure constant
intergalactic medium
quasar absorption lines

description: Extragalactic absorbers with column densities exceeding N<sub>H I</sub> &gt; 2 × 10<sup>20</sup> hydrogen atoms cm<sup>-2</sup> are generally termed damped Ly alpha (DLA) systems. The absorbers are basically comprised of predominantly neutral gas, which is an essential prerequisite for the formation of stars. The spectroscopy of absorbers offers the best possibility to study the chemical composition of the supposed equivalents of the interstellar medium of local galaxies at an earlier stage of cosmic evolution. Absorbers in the sub-DLA range with N<sub>H I</sub> &gt; 10<sup>19</sup> cm<sup>-2</sup> may consist of predominantly neutral gas, if the cosmic UV background is reduced. The observed kinematic structure of absorption lines as well as the column density and metallicty statistics of absorbers provide substantial tests for models of galaxy formation and cosmic structuring. Fine-structure lines offer the possibility to examine the temperature-redshift relation of the cosmic microwave background radiation predicted by Friedmann cosmology, or to infer the physical condition of the absorbing medium. Precise measurements of the relative positions of selected metal lines can impose stringent constraints on the cosmological variability of the fine structure constant alpha, which is an implication of many unifying theories currently discussed. The primary aim of this study is the examination of the ionization, abundance and dust depletion of metals in the sub-DLA absorber toward the QSO HE 0515-4414. The kinematic structure of metal absorption lines is discussed. Neutral carbon fine-structure lines are used to infer the temperature of the cosmic microwave background at redshift z = 1.15, and to diagnose the physical condition of the absorbing medium. Based on accurate positional measurements of individual components within an ensemble of selected Fe II resonance lines, the possible deviation of the fine structure constants from its current value is probed. High-resolution spectra obtained with the STIS and UVES spectrographs provide the empirical basis of this work. In the optical the quality of the data is exceptional. The program used for the analysis has been developed from scratch. The software is driven by an evolution strategy, which is based on a derandomized adaptation the covariance matrix. The strategy is proven to be a highly competitive algorithm capable of calculating an optimal line profile decomposition without requiring any particular initialization. The ionization conditions within the absorber have been analyzed using the simulation software Cloudy. The sub-DLA system with redshift z = 1.15 is unusual in several respects. The velocity interval of metal lines associated with the H I absorption extends for more than 700 km s<sup>-1</sup>, which is the second largest spread yet observed in this context. The extreme spread facillitates the unusual detection of two peripheral H I components located at distances of 530 and 240 km s<sup>-1</sup> from the N<sub>H I</sub> = 8 × 10<sup>19</sup> cm<sup>-2</sup> main component. The associated metal lines consist of at least 31 components. The kinematic distribution of metal line components is bimodal, which is possibly indicating the presence of two interacting absorbers. This interpretation is supported by the finding of a nonuniform ionization structure. The main absorber essentially consists of predominantly neutral gas, whereas the periphery is highly ionized. For the former, the abundances ratios of refractory elements to zinc range from Galactic warm disk to halo-like and essentially undepleted patterns. The dust-corrected metallicity of the main absorber is almost solar [Fe/H] = -0.08 ± 0.19, whereas the uncorrected metallicity is [Zn/H] = -0.38 ± 0.04. The element abundances in the periphery do conform with solar composition. The cosmic microwave background is only a minor source of the observed population of excited fine-structure levels. The excitation is predominantly caused by local FUV radiation and collisions with hydrogen atoms. The kinetic temperature of the absorbing medium is some ten to hundred Kelvin. The measured deviation of the fine-structure constant from its present value da/a = (0.1 ± 1.7) × 10<sup>-6</sup> does not confirm the detection of a cosmological variation found in the studies of John Webb and Michael Murphy. The present measurement has been affirmed by two further research teams. The present study shows that the chemical evolution of sub-DLA system toward HE 0515-4414 must have reached an advanced state. Since in this context regular DLA absorbers fall behind expectations at redshifts z &lt; 1.5, the question arises whether the sub-DLA absorber examined in the present work is more than just an exceptional case. In particular at lower redshifts the population of sub-DLA absorbers is largely unknown. Modern Surveys setting the detection limit below the traditional threshold of N<sub>H I </sub>&gt; 2 × 10<sup>20</sup> cm<sup>-2</sup> may provide interesting insights.
description: This research has been funded by the Verbundforschung of the BMBF/DLR under Grant No. 50 OR9911 1 and by the DFG under Re 353/48.
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