2011ApJ...736...42R

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 ∼10. We compile a statistical sample of 50 τ_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)∝(1 + z)^γ, with γ = 1.33±0.61 at z < 2.6, or with γ = 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 ∼1.5 over the ∼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 ∼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_) ∝ N ^–β_H I_) 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_ ≲ 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 ∼9 from z ∼ 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.