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Up: Metal abundances and kinematics


1 Introduction

Absorption systems in quasar spectra provide unique information on the intervening intergalactic matter (IGM) up to redshift $z \simeq 6$, back to the time when the Universe was less than 7% of its present age. High resolution spectroscopic observations available nowadays at large telescopes open new opportunities to investigate the physical nature of quasar absorbers. Reliable data on the chemical composition of the IGM and on the physical characteristics (like velocity and density distributions, volumetric gas density, kinetic temperature, ionization structure etc.) of the absorbers is an important clue to our understanding of galaxy formation, chemical evolution of the IGM and the origin of the large-scale structure.

In recent investigations much attention find the metal systems which are the absorbers exhibiting as a rule numerous lines of low (like H I, C II, Si II, Mg II, Al II) and high (like C III, N III, Si III, C IV, Si IV, N V) ionized species.

Presence of metals provides a unique opportunity to study the physical conditions of matter at early epochs. Unfortunately, the computational methods usually applied to high resolution spectra lie quite often behind the quality of observational data and fail to extract from them all encoded information. The common processing method consists of the deconvolution of complex absorption profiles into an arbitrary number of separate components (assuming a constant gas density within each of them) which are then fitted to Voigt profiles. However, in many cases this procedure may not correspond to real physical conditions: observed complexity and non-similarity of the profile shapes of different ions indicate that these systems are in general absorbers with highly fluctuating density and velocity fields tightly correlated with each other. Too high or too low gas temperatures, extremely varying metallicities between subcomponents, exotic UV background spectra and other physically badly founded outcomings may be artifacts arising from the inconsistency of the applied methods (see examples in Levshakov et al. 1999; Levshakov et al. 2000b, hereafter Paper I).

In Paper I we developed a new method for the QSO spectra inversion, - the Monte Carlo Inversion (MCI), - assuming that the absorbing region is a cloud with uniform metallicity but with fluctuating density and velocity fields inside it. This computational procedure which is based on stochastic optimization allows us to recover both the underlying hydrodynamical fields and the physical parameters of the gas. First application of the MCI to the analysis of the $z_{\rm abs} = 3.514$ system toward Q08279+5255 (Levshakov et al. 2000a) has shown that the proposed method is very promising especially in the inversion of complex absorption spectra with many metal lines.

In this paper we start a new comprehensive survey of the metal systems for which high resolution and high signal-to-noise spectra are available. We present here the results for three absorption systems ( $z_{\rm abs}$ = 1.87, 1.92 and 1.94) from the spectrum of the quasar J2233-606 which have been already studied by Prochaska & Burles (1999), and D'Odorico & Petitjean (2001, hereafter DP) using the common Voigt fitting method. We re-calculate these systems using the MCI in order to compare the applicability of both approaches and to show up their restrictions.

The structure of the paper is as follows. In Sect. 2 the data sets used in the MCI analysis are described. Section 3 contains the details of the applied computational procedure. The results obtained for each of the mentioned above systems are presented in Sect. 4. Conclusions are reported in Sect. 5. In Appendix the general equation of the entropy production rate is given which is used to calculate the density and velocity configurations along the line of sight exhibiting minimum dissipation.


  \begin{figure}
\par\includegraphics[height=13cm,width=16.5cm,clip]{H3203F1.PS}\end{figure} Figure 1: Hydrogen and metal absorption lines associated with the $z_{\rm abs} \simeq 1.92$ system toward J2233-606 (normalized intensities are shown by dots with $1\sigma $ error bars). The zero radial velocity is fixed at z = 1.92595. Smooth lines are the synthetic spectra convolved with the corresponding point-spread functions ( $FWHM_{\rm VLT} = 6.7$ km s-1, $FWHM_{\rm HST} = 10.0$ km s-1) and computed with the physical parameters from Table 1. Bold horizontal lines mark pixels included in the optimization procedure. The normalized $\chi ^2_{\rm min} = 1.10$ (the number of degrees of freedom $\nu = 1450$).


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