Intrinsic Properties of the ⟨z⟩ = 2.7 Lyα Forest from Keck Spectra of Quasar HS 1946+7658

We present the highest quality Lyα forest spectra published to date. We have complete 7.9 km s-1 FWHM spectra between the Lyα and Lyβ emission lines of the bright, high-redshift (V = 15.9, z = 3.05) QSO HS 1946+7658. The mean redshift of observed Lyα forest clouds is ⟨z⟩ = 2.7. The spectrum has a signal-to-noise ratio per pixel of 2 km s-1 that varies from 15 at 4190 Å to 100 at 4925 Å. The absorption lines in the spectrum have been fitted with Voigt profiles, and the distribution of Voigt parameters has been analyzed. We show that fitting Voigt profiles to high quality data does not give unique results. We have performed simulations to differentiate between true features of the line distributions and artifacts of line blending and the fitting process. We show that the distribution of H I column densities is a power law of slope -1.5 from N(H I) = 1014 cm-2 to N(H I) < 1012.1 cm-2. We further show that our data is consistent with the hypothesis that this power law extends to N(H I) = 0, because lines weaker than N(H I) = 1012.1 cm-2 do not have a significant H I optical depth. At velocity dispersions between 20 and 60 km s-1 the velocity dispersion (b) distribution is well described by a Gaussian with a mean of 23 km s-1, and a σb of 14 km s-1. Very similar N(H I) and b distributions were found at ⟨z⟩ = 3.7 by Lu et al. (1997), indicating no strong redshift evolution in these distributions for the Lyα forest. However, our b distribution has a lower mean and a wider dispersion than in past studies at the same redshift (e.g., Hu et al. 1995), which had lower signal-to-noise spectra. We unambiguously see narrow Lyα forest clouds with 14 km s-1 ≤ b ≤ 20 km s-1 that cannot be accounted for by noise effects. Our data also has absorption lines with b ≥ 80 km s-1 that cannot be explained by the blending of lower b lines. We find that the lower cutoff in the b distribution varies with N(H I), from b = 14 km s-1 at N(H I) = 1012.5 cm-2 to b = 22 km s-1 at N(H I) = 1014.0 cm-2. However, we see no similarly strong indication of a general correlation between b and N(H I). In contrast with previous results, we find no indication of Lyα forest line clustering on any scale above 50 km s-1. Even among lines with 1013.6 < N(H I) < 1014.3 cm-2, which were previously thought to cluster very strongly on velocity scales between 50 and 150 km s-1, we see no clustering on any scale above 50 km s-1, although we do see a 3 σ clustering signal between 25 and 50 km s-1 among these higher column density lines. With the distributions we have derived, we have calculated the expected He II optical depth of the Lyα forest. If there are no lines with N(H I) < 1012.1 cm-2, the Lyα forest is unlikely to provide a significant portion of the He II optical depth at high redshift. However, if the distribution extends to N(H I) < 109 cm-2, the Lyα forest can provide all of the observed optical depth if N(He II)/N(H I) ≈ 100. We have calculated the redshift evolution of the optical depth from the He II Lyα forest based upon the line distributions we have derived for the H I Lyα forest. If the Lyα forest is responsible for the high-redshift He II optical depth and the spectral shape of the UV background does not change with redshift, we predict τHe II ≈ 2.4 at z = 3.3 to be consistent with the value of τHe II previously found at z = 2.4, provided that Lyα forest lines with N(H I) < 1013 cm-2 evolve like those with N(H I) > 1013 cm-2.