Dissecting the Properties of Optically Thick Hydrogen at the Peak of Cosmic Star Formation History

Abstract

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 λ ~ 3000-4000 Å, we measure the number of LLSs per unit redshift ℓ(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 $\lambda ^{912}_{\rm mfp} = 100 \pm 29 \ h_{70.4}^{-1}\ {\rm Mpc}$. Combined with our LLS survey, we conclude that systems with log N H I ≥ 17.5 cm–2 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 H I ~ 17.5-19 cm–2 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 lsim 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 lsim 3 can be modeled either by requiring that the spatial extent of the circumgalactic medium is redshift invariant in the last ~10 Gyr of cosmic evolution or by postulating that LLSs arise in halos that are rare fluctuations in the density field at each redshift.