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 catalogued LLSs at z<2.6 by a factor of ~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) = C1 (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 ~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(HI)), at low redshift is not well
fitted by a single power law index (f(N(HI)) = C2 N(HI)^(-beta)) over the
column density range 13 < log N(HI) < 22 or log N(HI) >17.2. While low and high
redshift f(N(HI)) distributions are consistent for log N(HI)>19.0, there is
some evidence that f(N(HI)) evolves with z for log N(HI) < 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.
