THE DISTRIBUTION OF COLUMN DENSITIES AND B VALUES IN THE LYMAN-ALPHA FOREST

We describe the properties of the Ly alpha forest in the column density range N(H1)greater than or equal to 2 X 10(12) cm(-2) based on 1056 lines in the wavelength range 4300-5100 Angstrom 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 X 10(12) cm(-2) 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(-1) 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 Ly alpha absorption lines as complexes of clouds each with thermal broadening corresponding to b, 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(-1). (C) 1995 American Astronomical Society.