Monte Carlo inversion of hydrogen and metal lines from QSO absorption spectra

The primary objective of the study is the investigation of the physical properties and chemical composition of matter at different cosmological epochs. The investigation is based on high-resolution spectral observations of hydrogen and metal (C IV, Si IV, O VI and others) absorption-line profiles and advanced models of the radiative transfer in random media.

Currently, there are two methods to analyse absorption spectra:

  1. a conventional Voigt-profile fitting (VPF) procedure which usually assumes several subcomponents with their own physical parameters to describe a complex absorption profile, and

  2. a mesoturbulent approach which describes the line formation process in a continuous medium with fluctuating physical characteristics.

It is hard to favor this or that method if both of them provide good fitting. But the observed increasing complexity of the line profiles with increasing spectral resolution gives some preference to the model of the fluctuating continuous medium.

QSO velocity profiles We set forward a mesoturbulent approach to measure D/H and metal abundances, which has many advantages over the standard VPF procedures. A complete description of a new Monte Carlo inversion (MCI) method is given in Levshakov, Agafonova and Kegel (2000a,b).

An example of the MCI analysis of a "H+D"-like profile with accompanying metal lines observed at za=3.514 towards the quasar APM 08279+5255 is shown in Figure. The high quality spectral data have been obtained with the Keck-I telescope and the HIRES spectrograph by Ellison et al. (1999).

A VPF analysis of the primordial D/H in the za=3.514 system has been made by Molaro et al. (1999). They found D/H=1.5*10-5 in the cloud with N(HI)=1.2*1018 cm-2.

Using MCI, we minimize the c2 value. The objective function includes those pixels which are critical to the fit. In Figure, these pixels are marked by yellow. In Figure (panels f and c), the observed profiles of CIV1550 and, respectively, SiIV1402 are shown together with the model spectra computed with the parameters derived from Ly-alpha, CII1334, CIV1548, SiIII1206, and SiIV1393 fitting to illustrate the consistency. For the same reason the Ly-beta model spectrum is shown in panel b at the expected position. All model spectra in Figure are drawn by continuous curves, whereas filled circles represent observations (normalized fluxes). The corresponding distributions of v(x), n(x)/n0, and T(x) are shown in panels i, j, and k. The restored velocity field reveals a complex structure which is manifested in non-Gaussian density-weighted velocity distribution as shown in panels l and m for the total hydrogen as well as for the individual ions. We found that the radial velocity distribution of HI in the vicinity of v=-100 km/s may mimic the deuterium absorption and, thus, the asymmetric blue wing of the hydrogen Ly-alpha absorption may be readily explained by HI alone.

The median estimation of the model parameters gives N(H)=5.9*1018 cm-2 and N(HI)=5.3*1015 cm-2.

The MCI allowed us to fit precisely not only the blue wing of the saturated Ly-alpha line but the red one as well. We found that the actual neutral hydrogen column density may be a factor of 250 lower than the value obtained by Molaro et al. if one accounts for the velocity field structure. Besides we did not confirm the extremely low metallicity of [C/H]=-4.0, and [Si/H]=-3.5 reported by Molaro et al. Our analysis yields [C/H]=-1.8, and [Si/H]=-0.7. A similar silicon overabundance has also been observed in halo (population II) stars.

We may conclude that up-to-now deuterium was detected in only four QSO spectra (Q1937-1009, Q1009+2956, Q0130-4021, and Q1718+4807) where N(HI) was measured with a sufficiently high accuracy. These measurements are in concordance with D/H=(3-4)*10-5.


Ellison, S. L., Lewis, G. F., Pettini, M., Sargent, W. L. W., Chaffee, F. H., Foltz, C. B., Rauch, M., Irwin, M. J. 1999, PASP, 111, 919

Levshakov, S. A., Agafonova, I. I., Kegel, W. H. 2000a, A&A, 355, L1

Levshakov, S. A., Agafonova, I. I., Kegel, W. H. 2000b, A&A, 360, 833

Molaro, P., Bonifacio, P., Centurion, M., Vladilo, G. 1999, A&A, 349, L13