The low-redshift Lyα forest toward PKS 0405-123

We present results for Ly alpha forest and metal absorbers from similar to 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 Ly alpha forest lines, containing 60 absorbers with column density log N(H I) >= 13.3 over 0.002 < z < 0.423 and 44 with log N(H I) >= 13.1 over 0: 020 < z < 0: 234, respectively. Seven absorbers contain metals, all with associated O VI and often offset in velocity from Ly alpha. The strong and weak Doppler parameter distributions have < b > = 47 +/- 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 H I < 14.0, we find a redshift overdensity of similar to 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 Ly alpha-Ly alpha clustering in our sample on a scale of Delta v <= 250 km s(-1) for log N (H I) < 13.3, consistent with a numerical model of structure evolution. We detect velocity correlations of up to 250 km s(-1) between Ly alpha absorbers and 39 galaxies at z < 0.43 out to a transverse distance of 1.6 h(70)(-1) local frame Mpc. The Ly alpha-galaxy two-point correlation function is significant out to Delta v < 250 km s(-1) and grows with minimum H I column density. The strongest signal occurs for log N (H I) greater than or similar to 13.5-14.0 absorbers and is similar to the galaxy-galaxy correlation, implying that such Ly alpha absorbers have masses log (M/M(circle dot)) = 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.