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The Local Lyα Forest. II. Distribution of H I 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 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 for 12.5 ≤ log ≤ 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 ΩLyα = (0.008 ± 0.001)h, 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.