The local Lyα forest.: II.: Distribution of HI absorbers, Doppler widths, and baryon content

In Paper I of this series we described observations of 15 extragalactic targets taken with the Hubble Space Telescope+ GHRS + G160M grating for studies of the low-z Ly alpha forest. We reported the detection of 111 Ly alpha absorbers at significance level (SL) greater than or equal to3 sigma, 81 with SL greater than or equal to 4 sigma, in the redshift range 0.002 < z < 0.069, over a total path length c Deltaz = 116,000 km s(-1) (Deltaz = 0.387). In this second paper, we evaluate the physical properties of these Lya absorbers and compare them to their high-z counterparts. The SL greater than or equal to 4 a distribution of Doppler parameters is similar to that at high redshift, with <b > = 38.0 +/- 15.7 km s(-1). The true Doppler parameter may be somewhat lower, owing to component blends and nonthermal velocities. The distribution of equivalent widths exhibits a significant break at W less than or equal to 133 m Angstrom, with an increasing number of weak absorbers (10 m Angstrom < W < 100 m Angstrom). Adopting a curve of growth with b = 25 +/- 5 km s(-1) and applying a sensitivity correction as a function of equivalent width and wavelength, we derive the distribution in column density, N-HI(-1.72+/-0.06) for 12.5 less than or equal to log [N-HI] less than or equal to 14.0. We find no redshift evolution within the current sample at z < 0.07, but we do see a significant decline in d N /dz compared to values at z > 1.6. Similar to the high equivalent width (W > 240 m Angstrom) absorbers, the number density of low-W absorbers at z = 0 is well above the extrapolation of d N /dz from z > 2, but we observe no difference in the mean evolution of dN/dz between absorbers of high (W > 240 m Angstrom) and low (W less than or equal to 100 m Angstrom) equivalent width. While previous work has suggested slower evolution in number density of lower W absorbers, our new data do not support this conclusion. A consistent evolutionary pattern is that the slowing in the evolution of the low column density clouds occurs at lower redshift than for the higher column density clouds. A 4-5 a signal in the two-point correlation function of Ly alpha absorbers for velocity separations Deltav less than or equal to 150 km s(-1) is consistent with results at high z, but with somewhat greater amplitude. Applying a photoionization correction, we find that the low-z Ly alpha forest may contain similar to 20% of the total number of baryons, with closure parameter OmegaL(y alpha) = (0.008 +/- 0.001)h(70)(-1), for a standard absorber size and ionizing radiation held. Some of these clouds appear to be primordial matter, owing to the lack of detected metals (Si III) in a composite spectrum, although current limits on composite metallicity are not strong.