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 a megaparsec-scale intergalactic sheet (pancake) at z similar to (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, similar to(7.7 x 10(6)-1.5 x 10(4))M (circle dot), one of the highest-resolution simulations of such a large patch of the intergalactic medium (IGM) to date. At z similar to 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(-3) Z (circle dot)) Lyman-limit systems (N (H I) > 10(17.2)cm(-2)) in the z similar to 4 IPM increases from similar to(3-15)%, while that of metal-poor damped Ly alpha absorbers (N (H I) > 10(20)cm(-2)) increases from similar to(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 (s) t (cool), at T similar to 10(5) K. If this is unresolved, gas piles up at T less than or similar to 10(5) 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.

Automated summary

This study utilizes novel cosmological simulations to investigate the thermal properties and H i content of the warm-hot intergalactic medium at high redshift, finding the formation of a strong accretion shock around the intergalactic pancake. The research also reveals the presence of unconverged mass, morphology, and distribution of H i in the warm-hot intergalactic medium even at the highest resolution.