# Shattering of Cosmic Sheets due to Thermal Instabilities: a Formation Channel for Metal-Free Lyman Limit Systems

## TL;DR

This paper uses high-resolution cosmological simulations to reveal how thermal instabilities in large-scale structure lead to the formation of metal-free Lyman limit systems outside galaxies, providing a new formation channel.

## Contribution

It introduces a novel simulation approach showing the formation of pristine, cold gas clouds via sheet shattering due to thermal instabilities in the intergalactic medium.

## Key findings

- Cold clouds have N_{HI}>10^{17.2}cm^{-2} and are detectable as Lyman limit systems.
- Clouds are metal-free and located outside galaxy halos.
- Sheet shattering occurs from merger-induced shocks leading to thermal instabilities.

## Abstract

We present a new cosmological zoom-in simulation, where the zoom region consists of two halos with virial mass M_v~5x10^{12}M_{sun} and a ~Mpc long cosmic filament connecting them at z~2. Using this simulation, we study the evolution of the intergalactic medium in between these two halos at unprecedented resolution. At 5>z>3, the two halos are found to lie in a large intergalactic sheet, or "pancake", consisting of multiple co-planar dense filaments along which nearly all halos with M_v>10^9M_{sun} are located. This sheet collapses at z~5 from the merger of two smaller sheets. The strong shock generated by this merger leads to thermal instabilities in the post-shock region, and to a shattering of the sheet resulting in <~kpc scale clouds with temperatures of T>~2x10^4K and densities of n>~10^{-3}cm^{-3}, which are pressure confined in a hot medium with T~10^6K and n>~10^{-5}cm^{-3}. When the sheet is viewed face on, these cold clouds have neutral hydrogen column densities of N_{HI}>10^{17.2}cm^{-2}, making them detectable as Lyman limit systems, though they lie well outside the virial radius of any halo and even well outside the dense filaments. Their chemical composition is pristine, having zero metalicity, similar to several recently observed systems. Since these systems form far from any galaxies, these results are robust to galaxy formation physics, resulting purely from the collapse of large scale structure and radiative cooling, provided sufficient spatial resolution is available.