Thermal Instabilities and Shattering in the High-redshift WHIM: Convergence Criteria and Implications for Low-metallicity Strong H i Absorbers

Using a novel suite of cosmological simulations zooming in on Mpc-scale intergalactic sheet or "pancake" at z~3-5, we conduct an in-depth study the thermal properties and HI content warm-hot medium (WHIM) those redshifts. The span nearly three orders magnitude gas-cell mass, from ~(7.7x10^6-1.5x10^4)Msun, one highest resolution such large patch inter-galactic (IGM) to date. At z~5, strong accretion shock develops around main pancake following collision between two smaller sheets. Gas post-shock region proceeds cool rapidly, triggering instabilities formation multiphase medium. We find neither nor morphology, distribution WHIM be converged our resolution. Interestingly, lack convergence is more severe for less dense, metal-poor, intra-pancake (IPM) filaments far any star-forming galaxies. As increases, IPM shattered structure, with ~kpc scale clouds containing most HI. From lowest resolution, covering fraction metal-poor (Z<10^{-3}Zsun) Lyman-limit systems (NHI>10^{17.2}/cm^2) z~4 increases (3-15)%, while that Damped Lyman-alpha Absorbers (NHI>10^{20}/cm^2) similar metallicity threefold, (0.2-0.6)%, no sign convergence. necessary condition multiphase, structure resolving cooling length, lcool=cs*tcool, T~10^5K. If this unresolved, gas "piles up" these temperatures lower becomes very inefficient. conclude state-of-the-art are still unable resolve multi-phase low-density IGM, potentially far-reaching implications.