The Distribution of Column Densities and b Values in the Lyman - Alpha Forest

We describe the properties of the Lyα forest in the column density range NH I ≥ 2× 10 12 cm based on 1056 lines in the wavelength range 4300− 5100 Å measured in extremely high S/N, R = 36, 000 spectra of four quasars. The column density distribution is well described by a −1.5 power law to 2×10 cm, below which limit confusion becomes too severe to measure a spectrum of individual clouds. The distribution of b values shows a well-defined lower envelope with a cutoff at b = 20 km s corresponding to a cloud temperature of 24,000 K. There is only a very small fraction (less than 1%) of narrow line clouds which cannot be identified with metal-lines. From modeling the Lyα absorption lines as complexes of clouds each with thermal broadening corresponding to bc we find the b distribution can be understood if there is a mean of 3.25 clouds per absorption line with a spread in velocity centroids characterized by a dispersion of 10.75 km s. Subject headings: cosmology: early universe — cosmology: observations — galaxies: intergalactic medium — galaxies: quasars: absorption lines — galaxies: quasars: individual (0014+813 = S5 0014+81) — galaxies: quasars: individual (0302–003) — galaxies: quasars: individual (0636+680 = S4 0636+68) — galaxies: quasars: individual (0956+122) Visiting Astronomer, W. M. Keck Observatory, jointly operated by the California Institute of Technology and the University of California.