The local Ly alpha forest. II. Distribution of HI absorbers, Doppler widths, and baryon content

In Paper I of this series we described observations of 15 extragalactic tar gets taken with the Hubble Space Telescope+ GHRS + G160M grating for studie s of the low-z Ly alpha forest. We reported the detection of 111 Ly alpha a bsorbers at significance level (SL) greater than or equal to3 sigma, 81 wit h 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 abs orbers and compare them to their high-z counterparts. The SL greater than o r equal to 4 a distribution of Doppler parameters is similar to that at hig h 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 le ss than or equal to 133 m Angstrom, with an increasing number of weak absor bers (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 o f equivalent width and wavelength, we derive the distribution in column den sity, N-HI(-1.72+/-0.06) for 12.5 less than or equal to log [N-HI] less tha n 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 va lues 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 m ean evolution of dN/dz between absorbers of high (W > 240 m Angstrom) and l ow (W less than or equal to 100 m Angstrom) equivalent width. While previou s work has suggested slower evolution in number density of lower W absorber s, our new data do not support this conclusion. A consistent evolutionary p attern is that the slowing in the evolution of the low column density cloud s 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 v elocity 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 ph otoionization correction, we find that the low-z Ly alpha forest may contai n similar to 20% of the total number of baryons, with closure parameter Ome gaL(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 matt er, owing to the lack of detected metals (Si III) in a composite spectrum, although current limits on composite metallicity are not strong.