Dissecting the properties of optically-thick hydrogen at the peak of cosmic star formation history

TL;DR

This study investigates the properties and distribution of Lyman limit systems at z~3, revealing their contribution to the universe's mean free path of ionizing photons and implications for galaxy halo gas and intergalactic medium evolution.

Contribution

It provides the first blind survey of LLSs at z~3, measures their number density, mean free path, and models their origins in galaxy halos and the intergalactic medium.

Findings

LLSs contribute ~40% to the mean free path at z~3.

Number of LLSs per unit redshift is roughly constant at z<3.

Gas in galaxy halos can account for all LLSs at z<3.

Abstract

We present 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 ~3000-4000A, 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 = 100+/-29 Mpc/h_70.4. Combined with our LLS survey, we conclude that systems with log N_HI >= 17.5 cm^-2 contribute only ~40% to the observed mean free path at these redshifts. Further, with the aid of photo-ionization 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 N_HI = 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<~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 ~10 Gyr of cosmic evolution or by postulating that LLSs arise in halos that are rare fluctuations in the density field at each redshift.