Lyα absorption systems and the nearby galaxy distribution

We study the galaxy number density (smoothed on a 5 h(-1) Mpc scale) around 18 low-redshift Ly alpha absorbers previously observed with HST. The absorbers lie in the foregrounds of Mrk 335, Mrk 421, Mrk 501, I Zw 1, and 3C 273, all within regions where there are now complete redshift surveys to m(zw) = 15.5. We construct a smoothed galaxy number density held from the redshift survey data and determine the distribution of densities at the Ly alpha absorber locations. We also find the distribution of galaxy number density for a variety of test samples: all galaxy locations within the Center for Astrophysics Redshift Survey (CfA2), CfA2 galaxy locations along randomly selected lines of sight, and randomly chosen redshifts along random lines of sight. The Ly alpha absorbers are present in dense regions of the survey, but occur far more frequently in underdense regions than do typical luminous galaxies. The distribution of smoothed galaxy density around the Ly alpha absorbers is inconsistent at the 4 sigma level with the density distribution around survey galaxies. It is highly consistent with a density distribution at randomly chosen redshifts along random lines of sight. This supports earlier evidence that the nearby, low column density (log N(HI) less than or similar to 14) Ly alpha forest systems are spatially distributed at random; they are not well correlated with the local large-scale structure.