Astron. Astrophys. 336, L29-L32 (1998)

1. Introduction

From recent HST observations of a low-redshift () absorption-line system toward the quasar Q 1718+4807 () Webb et al. (1997a,b) deduced D/H = . This ratio is significantly higher than that derived from other quasar spectra at [D/H = by Tytler & Burles, 1996; D/H = by Levshakov, Kegel & Takahara, 1997 (LKT, hereinafter)], and at [D/H = by Tytler et al., 1996; D/H by Songaila et al., 1997].

The apparent spread of the D/H values leads some authors to assume fluctuations in the baryon-to-photon ratio at the epoch of BBN (see e.g. Webb et al., and references cited therein). On the other hand, according to the basic idea of homogeneity and isotropy of big bang theory the primordial deuterium abundance should not vary in space. One can only expect that the D/H ratio decreases with cosmic time due to conversion of D into ³He and heavier elements in stars. To check whether big bang nucleosynthesis has occurred homogeneously or not, precise measurements of absolute values of D/H at high redshift are extremely important. The fundamental character of this cosmological test requires an unambiguous interpretation of spectral observations.

It is well known, however, that the physical parameters derived from spectral data depend on the assumptions made with respect to the line broadening mechanism. For intergalactic absorption lines a `non-thermal broadening' is usually assumed to be caused by large scale motions of the absorbing gas. The commonly used microturbulent approach disregards all correlations of the velocity field, implying a symmetrical (Gaussian) distribution of the velocity components parallel to the line of sight and a symmetrical line profile.

Actually, any turbulent flow exhibits an immanent structure in which the velocities in neighboring volume elements are correlated with each other. Different aspects of the line formation processes in correlated turbulent media have been discussed recently in a series of papers by Levshakov & Kegel (1997, LK hereinafter), Levshakov, Kegel & Mazets (1997, LKM hereinafter), and by LKT.

Once the statistical properties of a turbulent cloud have been specified, the way spectral lines ought to be calculated, depends on the problem considered. Considering emission lines one is dealing with many lines of sight and the observed intensity should closely correspond to the theoretical expectation value (see e.g. Albrecht & Kegel 1987). If, however, one observes the absorption spectrum in the light of a point-like background source, the actually observed line profile is determined by the velocity distribution along the particular line of sight. Therefore, the intensity may deviate substantially from the expectation value, since averaging along one line of sight corresponds to averaging over an incomplete sample (for details see LK and LKM). For large values of the ratio of the cloud thickness L to the correlation length l the distribution function for the velocity component parallel to the line of sight approaches the statistical average, which has been assumed to be a Gaussian. For values of of only a few, however, may deviate substantially from a Gaussian, and is asymmetric in general. This leads to a complex shape of the absorption coefficient for which the assumption of Voigt profiles could be extremely misleading. The actual D/H ratio may turn out to be higher or lower than the value obtained from the Voigt-fitting procedure.

The present Letter is primarily aimed at the inverse problem in the analysis of the H+D Ly absorption observed by Webb et al. (1997a,b). The original analysis was performed in the framework of the microturbulent model. Here we make an attempt to re-analyze the observational data on the basis of a more general mesoturbulent model. We consider a cloud with a stochastic velocity field with finite correlation length but of homogeneous (H I -) density and temperature. The velocity field is characterized by its rms amplitude and its correlation length l. The model is identical to that of LKT. - The objective is to investigate whether the data in question may also be interpreted by a lower D/H ratio consistent with the values found for other absorption systems.

© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998
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