The low-redshift Lyα forest toward PKS 0405-123

We present results for Lyα forest and metal absorbers from ∼7 km s^-1 resolution Space Telescope Imaging Spectrograph data for QSO PKS 0405-123 (z = 0.574). We analyze strong and weak samples of Lyα forest lines, containing 60 absorbers with column density log N_H1 ≥ 13.3 over 0.002 < z < 0.423 and 44 with log N_H1 ≥ 13.1 over 0.020 < z < 0.234, respectively. Seven absorbers contain metals, all with associated O VI and often offset in velocity from Lyα. The strong and weak Doppler parameter distributions have 〈b〉 = 41 ± 22 and 44 ± 21 km s^-1, respectively. Simulated spectra suggest that line blending and signal-to-noise ratio effects inflate b. For absorbers with 13.1 < log N_HI, < 14.0, we find a redshift overdensity of ∼0.2-0.3 dex at 0.127 < z < 0.234, which we attribute to cosmic variance. There is a void in the strong sample at 0.0320 < z < 0.0814 with probability of random occurrence P < 0.0004. We detect Lyα-Lyα clustering in our sample on a scale of Δ_v ≤ 250 km s ^-1 for log N_HI ≥ 13.3, consistent with a numerical model of structure evolution. We detect velocity correlations of up to 250 km s^-1 between Lya absorbers and 39 galaxies at z < 0.43 out to a transverse distance of 1.6 h₇₀^-1 local frame Mpc. The Lyα-galaxy two-point correlation function is significant out to Δ_v < 250 km s^-1 and grows with minimum H I column density. The strongest signal occurs for log N_HI ≳ 13.5-14.0 absorbers and is similar to the galaxy-galaxy correlation, implying that such Lyα absorbers have masses log(M/M_Ȯ) = 11.3 _-0.6^+1.0. We find a correlation between local galaxy counts and local summed H I column density, with peak significance on scales of 4000-6000 km s^-1 and probability of random occurrence P = 0.0009. Finally, we present column densities for a number of Galactic species.