The Redshift Distribution of Ly α Forest Lines in Spectra of QSOs

Numerous narrow absorption lines in the region of wavelength shorter than 1216(1+zem) (zem is the emission redshift), i.e. so-called Ly α forest lines, detected in QSO spectra are usually thought to be produced in intervening primeval clouds. The study of Ly α clouds may reveal how matter distributes in space and how it evolves with time at the early universe and provide valuable information about the large scale structure of the universe and its evolution. Based on intermediate resolution (1 ∼ 2 A) spectra, many authors (e.g. Lu et al. 1991) deduced that the evolutionary index γ ∼ 2, (dN/dz ∼ (1 + z)γ, dN/dz is the number of clouds per unit redshift interval at redshift z). It means that Ly α clouds have strong cosmological evolution. In recent years, there appear high-resolution (< 30km/sec) spectra of QSOs. High resolution spectra may provide more information than medium resolution spectra. Hence, it is necessary to study the evolution of Ly α clouds, using the spectra with higher resolution. Carswell et al. (1987) found γ=1 in the redshift interval 1.9–3.8. But Rauch et al. (1992) found γ=2.1 for the line sample with logN(HI) ≥ 13.75. It is more interesting that Giallongo (1991) found a differential evolution: γ is depended on the equivalent width W of line and no evolution for the strong line sample with 0.5 > W > 0.3. However, these studies involved very few QSOs (three or four). In this paper, we use a larger sample of QSOs to study the evolution of Ly α clouds.