A HUBBLE SPACE TELESCOPE STUDY OF LYMAN LIMIT SYSTEMS: CENSUS AND EVOLUTION

We present a survey for optically thick Lyman limit absorbers at z < 2.6 using archival Hubble Space Telescope observations with the Faint Object Spectrograph and Space Telescope Imaging Spectrograph. We identify 206 Lyman limit systems (LLSs), increasing the number of cataloged LLSs at z < 2.6 by a factor of similar to 10. We compile a statistical sample of 50 tau(LLS) >= 2 LLSs drawn from 249 QSO sight lines that avoid known targeting biases. The incidence of such LLSs per unit redshift, l(z) = dn/dz, at these redshifts is well described by a single power law, l(z) proportional to (1 + z)(gamma), with gamma = 1.33 +/- 0.61 at z < 2.6, or with gamma = 1.83 +/- 0.21 over the redshift range 0.2 <= z <= 4.9. The incidence of LLSs per absorption distance, l(X), decreases by a factor of similar to 1.5 over the similar to 0.6 Gyr from z = 4.9 to 3.5; l(X) evolves much more slowly at low redshifts, decreasing by a similar factor over the similar to 8 Gyr from z = 2.6 to 0.25. We show that the column density distribution function, f(N-H I), at low redshift is not well fitted by a single power-law index (f(N-H I) proportional to N-H I(-beta)) over the column density range 13 <= log N-H I <= 22 or log N-H I >= 17.2. While low-and high-redshift f(N-H I) distributions are consistent for log N-H I > 19.0, there is some evidence that f(N-H I) evolves with z for log N-H I less than or similar to 17.7, possibly due to the evolution of the UV background and galactic feedback. Assuming LLSs are associated with individual galaxies, we show that the physical cross section of the optically thick envelopes of galaxies decreased by a factor of similar to 9 from z similar to 5 to 2 and has remained relatively constant since that time. We argue that a significant fraction of the observed population of LLSs arises in the circumgalactic gas of sub-L-* galaxies.