Shattering of Cosmic Sheets due to Thermal Instabilities: A Formation Channel for Metal-free Lyman Limit Systems

We present a new cosmological zoom-in simulation, where the zoom region consists of two halos with virial mass Mv ∼ 5 × 1012M⊙ and an approximately megaparsec 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 coplanar dense filaments along which nearly all halos with Mv > 109M⊙ 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 postshock region, and to a shattering of the sheet resulting in ≲ kiloparsec-scale clouds with temperatures of T ≳ 2 × 104 K and densities of n ≳ 10−3 cm−3, which are pressure confined in a hot medium with T ∼ 106 K and n ≳ 10−5 cm−3. When the sheet is viewed face-on, these cold clouds have neutral hydrogen column densities of NH i > 1017.2 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 metallicity, 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.