The Local Lyα Forest. II. Distribution of HiAbsorbers, 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 110 Lyα absorbers at significance level ≥ 3σ in the redshift range 0.002 z 0.069, over a total pathlength c∆z = 116,000 km s −1 (∆z = 0.387). In this second paper, we evaluate the physical properties of these Lyα absorbers and compare them to their high-z counterparts. The distribution of Doppler parameters is similar to that at high redshift, with 〈bobs 〉 = 35.0 ± 16.6 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 m˚A, with an increasing number of weak absorbers (10 m˚A W 100 m˚A). 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, NHI −1.80±0.05 for NHI 10 14 cm −2. We find no redshift evolution in the sample at z 0.07, but we do see a significant decline in dN/dz compared to values at z 1.6. A 3σ signal in the two-point correlation function of Lyα absorbers for velocity separations ∆v ≤ 150 km s −1 is consistent with results at high-z. 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 −1 70, 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. Our data suggest that a fraction of the absorbers are associated with gas in galaxy associations (filaments), while a second population is distributed more uniformly. Subject headings: intergalactic medium — quasars: absorption lines — ultraviolet: galaxies