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Description
We present the results of a blind survey of Lyman limit systems (LLSs) detected in absorption against 105 quasars at z~3 using the blue sensitive MagE spectrograph at the Magellan Clay telescope. By searching for Lyman limit absorption in the wavelength range {lambda}~3000-4000{AA}, we measure the number of LLSs per unit redshift {ell}(z)=1.21+/-0.28 at z~2.8. Using a stacking analysis, we further estimate the mean free path of ionizing photons in the z~3 universe {lambda}_mfp_^912^=100+/-29h_70.4_^-1^Mpc. Combined with our LLS survey, we conclude that systems with logN_HI_>=17.5/cm2 contribute only ~40% to the observed mean free path at these redshifts. Furthermore, with the aid of photoionization modeling, we infer that a population of ionized and metal poor systems is likely required to reproduce the metal line strengths observed in a composite spectrum of 20 LLSs with log N_HI_~17.5-19/cm2 at z~2.6-3.0. Finally, with a simple toy model, we deduce that gas in the halos of galaxies can alone account for the totality of LLSs at z<~3, but a progressively higher contribution from the intergalactic medium is required beyond z~3.5. We also show how the weakly evolving number of LLSs per unit redshift at z<~3 can be modeled either by requiring that the spatial extent of the circumgalactic medium is redshift invariant in the last ~10Gyr of cosmic evolution or by postulating that LLSs arise in halos that are rare fluctuations in the density field at each redshift.
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