Intrinsic Properties of the < z > = 2 . 7 Lyman Alpha Forest from Keck Spectra of QSO 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 4190Å to 100 at 4925Å. The absorption lines in the spectrum have been fit 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) = 10 14 cm −2 to N(H I) < 10 12.1 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) = 10 12.1 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 σ b 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 (eg. Hu et al. 1995) which had lower signal to noise spectra. We unambiguously see narrow – 2 – 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 can not 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) = 10 12.5 cm −2 to b = 22 km s −1 …