Cold Stream Penetration of Virial Shocks: Fragmentation, Coagulation, and Disruption in the Hot Circumgalactic Medium

Cold streams penetrating virial shocks of massive halos along cosmic web filaments are expected to fuel galaxy growth at high redshift, yet the physical processes governing their penetration remain uncertain. We investigate cylindrical cold streams penetrating a hot circumgalactic medium (CGM) using idealized three-dimensional simulations. We systematically vary the stream radius, Mach number, and initial stream-to-CGM pressure contrast across three density contrasts, while controlling stream properties after pressure equilibrium is re-established. We identify three evolutionary regimes: coagulation, fragmentation, and disruption, plus a borderline regime in which the stream core marginally survives while detached fragments are disrupted. At modest pressure contrast, survival depends primarily on the competition between velocity shear and radiative cooling. Increasing pressure contrast produces a transient response during pressure restoration, temporarily enhancing or suppressing cold-gas mass and cold-hot interfacial area before evolution converges to a shear-dominated state. At larger pressure contrasts, the oblique shock steepens into a bow shock, and the final outcome is determined by the ratio of post-shock cooling time to virial crossing time. In all survival cases, post-equilibration evolution is well described by turbulent radiative entrainment at the stream-CGM interface: cold-gas mass flux increases while mean streamwise momentum flux remains approximately conserved. Applying these results to a galaxy evolution framework, we find that cold streams are expected to penetrate virial shocks around massive halos at z > 2, though they may fragment into multiphase structures within the hot CGM at Mv ≳ 2 × 1012 M⊙. Toward lower redshift, bow-shock heating and inefficient post-shock cooling can suppress cold-stream penetration.