The Ly forest of the quasar HS 1946 + 7658: properties of the Ly absorbing systems at high z

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Abstract
The optical spectrum of the QSO HS 1946 + 7658 (zem = 3.05) has been observed at high resolution (b ∼ 14 km s −1) in the wavelength interval 4120–4970 Å. The high signal-to-noise ratio ( ≥ 10) in the Lyα forest allows the determination of the neutral hydrogen column density as well as the velocity dispersion of the intervening clouds by fitting Voigt profiles. These results have been included in a global sample of Lyα lines from six QSOs ranging from redshift 2.14 to 4.51, at a similar signal-to-noise ratio and resolution selected from the literature (654 lines satisfying our criteria). The study of this sample confirms a plateau and a strong fall-off (cut-off point at log NHI ≃ 14) in the NHI distribution. The b distributions above and below such a cutoff show similar asymmetric bell-shaped forms but significantly different mean and median values (‹b› = 28, 36 km s−1 and bmed = 32, 37 km s−1, for weak and strong lines respectively), arguing against the independence of b and NHI. Supporting such a relation, the b-NHI diagram suffers a deficit of strong lines with low b values (K-S probability of the data coming from uncorrelated distributions pK-S = 0.047). We have found that the number density of systems per unit interval of redshift can be fitted to a power law with index γ = 2.35 ± 0.22 when all the QSOs are included, but a higher slope γ = 3.22 ± 0.41 is obtained for the redshift range 2.1 < z z for weak (log NHI 14) ones, typical b values decrease with z.

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