2000ApJ...544..150P

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 Lyα forest. We reported the detection of 111 Lyα absorbers at significance level (SL) ≥3 σ, 81 with SL≥4 σ, in the redshift range 0.002<z<0.069, over a total path length cΔz=116,000 km.s^–1 (Δz=0.387). In this second paper, we evaluate the physical properties of these Lyα absorbers and compare them to their high-z counterparts. The SL≥4 σ 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≤133 mÅ, with an increasing number of weak absorbers (10 mÅ<W<100 mÅ). 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^{–1.72±0.06}_HI_ for 12.5≤logN_HI_≤14.0. We find no redshift evolution within the current sample at z<0.07, but we do see a significant decline in dN/dz compared to values at z>1.6. Similar to the high equivalent width (W>240 mÅ) absorbers, the number density of low-W absorbers at z=0 is well above the extrapolation of dN/dz from z>2, but we observe no difference in the mean evolution of dN/dz between absorbers of high (W>240 mÅ) and low (W≤100 mÅ) 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 σ signal in the two-point correlation function of Lyα absorbers for velocity separations Δv≤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 Lyα forest may contain ∼20% of the total number of baryons, with closure parameter Ω_Lyα=(0.008±0.001)h^–1₇₀, for a standard absorber size and ionizing radiation field. 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.