DISSECTING THE PROPERTIES OF OPTICALLY THICK HYDROGEN AT THE PEAK OF COSMIC STAR FORMATION HISTORY

We present the results of a blind survey of Lyman limit systems (LLSs) detected in absorption against 105 quasars at z similar to 3 using the blue sensitive MagE spectrograph at the Magellan Clay telescope. By searching for Lyman limit absorption in the wavelength range lambda similar to 3000-4000 angstrom, we measure the number of LLSs per unit redshift l(z) = 1.21 +/- 0.28 at z similar to 2.8. Using a stacking analysis, we further estimate the mean free path of ionizing photons in the z similar to 3 universe lambda(912)(mfp) = 100 +/- 29 h(70.4)(-1) Mpc. Combined with our LLS survey, we conclude that systems with log N-HI >= 17.5 cm(-2) contribute only similar to 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 similar to 17.5-19 cm(-2) at z similar to 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 less than or similar to 3, but a progressively higher contribution from the intergalactic medium is required beyond z similar to 3.5. We also show how the weakly evolving number of LLSs per unit redshift at z less than or similar to 3 can be modeled either by requiring that the spatial extent of the circumgalactic medium is redshift invariant in the last similar to 10 Gyr of cosmic evolution or by postulating that LLSs arise in halos that are rare fluctuations in the density field at each redshift.