The Local Lyα Forest. II. Distribution of HiAbsorbers, Doppler Widths, and Baryon Content

In Paper I of this series (astro-ph/9911117) we described observations of 15extragalactic targets taken with the Hubble Space Telescope GHRS/G160M gratingfor studies of the low-z Lya forest. We reported the detection of 110 Lyaabsorbers at significance level >3 sigma in the redshift range z=0.002-0.069,over a total pathlength of 116,000 km/s. In this second paper, we evaluate thephysical properties of these Lya absorbers and compare them to their high-zcounterparts. The distribution of Doppler parameters is similar to that at highredshift, with mean b = 35.0 +- 16.6 km/s. The true Doppler parameter may besomewhat lower, owing to component blends and non-thermal velocities. Thedistribution of equivalent widths exhibits a significant break at W~133mA, withan increasing number of weak absorbers (10mA-100mA). Adopting a curve of growthwith b = 25 +- 5km/s and applying a sensitivity correction as a function ofequivalent width and wavelength, we derive the distribution in column density,Nh^{-1.72+-0.06} for Nh<10^14 cm^-2. We find no redshift evolution in thesample at z<0.07, but we do see a significant decline in dN/dz compared tovalues at z>1.6. A 3 sigma signal in the two-point correlation function of Lyaabsorbers for velocity separations Delta v <150 km/s is consistent with resultsat high-z. Applying a photoionization correction, we find that the low-z Lyaforest may contain ~20% of the total number of baryons, with closure parameterOmega_lya = (0.008+-0.001), for a standard absorber size and ionizing radiationfield. Some of these clouds appear to be primordial matter, owing to the lackof detected metals in a composite spectrum. Our data suggest that a fraction ofthe absorbers are associated with gas in galaxy associations (filaments), whilea second population is distributed more uniformly.Comment: 59 pages, 22 figures, 7 tables, ApJ accepted, scheduled to appear Dec 1, 2000. Changes to the GHRS linelist (astro-ph/9911117) are reflected in this reviso