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Abstract

Using a novel suite of cosmological simulations zooming in on a megaparsec-scale intergalactic sheet (pancake) at z ∼ (3–5), we conduct an in-depth study of the thermal properties and H I content of the warm-hot intergalactic medium (WHIM) at those redshifts. The simulations span nearly three orders of magnitude in gas cell mass, ∼(7.7 × 10⁶–1.5 × 10⁴)M_⊙, one of the highest-resolution simulations of such a large patch of the intergalactic medium (IGM) to date. At z ∼ 5, a strong accretion shock develops around the pancake. Gas in the postshock region proceeds to cool rapidly, triggering thermal instabilities and generating a multiphase medium. We find the mass, morphology, and distribution of H I in the WHIM to all be unconverged, even at our highest resolution. Interestingly, the lack of convergence is more severe for the less-dense, metal-poor intrapancake medium (IPM) in between filaments and far outside galaxies. With increased resolution, the IPM develops a shattered structure with most of the H I in kiloparsec-scale clouds. From our lowest-to-highest resolution, the covering fraction of metal-poor (Z < 10⁻³Z_⊙) Lyman-limit systems (N_{H I} > 10^17.2cm⁻²) in the z ∼ 4 IPM increases from ∼(3–15)%, while that of metal-poor damped Lyα absorbers (N_{H I} > 10²⁰cm⁻²) increases from ∼(0.2–0.6)%, with no sign of convergence. We find that a necessary condition for the formation of a multiphase shattered structure is resolving the cooling length, l_cool = c_st_cool, at T ∼ 10⁵ K. If this is unresolved, gas "piles up" at T ≲ 10⁵ K and further cooling becomes very inefficient. We conclude that state-of-the-art cosmological simulations are still unable to resolve the multiphase structure of the WHIM, with potentially far-reaching implications.