Movie companion — Simulation gallery

Effects of Cloud Geometry and Metallicity on Shattering and Coagulation of Cold Gas, and Implications for Cold Streams Penetrating Virial Shocks

Zhiyuan Yao, Nir Mandelker, S. Peng Oh, Han Aung, Avishai Dekel

Racah Institute of Physics, The Hebrew University of Jerusalem · UC Santa Barbara · UC Santa Cruz / SCIPP

Published in MNRAS 536, 3053 (2025) · DOI: 10.1093/mnras/stae2771 · arXiv:2410.12914

Paper (MNRAS / DOI) ADS arXiv YouTube playlist Author homepage Stream-penetration companion

This page collects the simulation movies referenced in the paper above. The movies illustrate the thermal fragmentation (“shattering”) and coagulation of pressure-perturbed cold clouds in three idealised geometries — sheets, streams, and spheres. The page is intended as a visual supplement; for the physical analysis, parameter scans, and analytic modelling, please see the paper itself.

What you are looking at

Each clip is a snapshot sequence from a 3D Eulerian AMR simulation (RAMSES) of an initially cold, dense cloud embedded in a hot, dilute background and held in pressure equilibrium. A small density perturbation seeds an implosion driven by radiative cooling; the cloud rebounds (the “explosion” phase) and either fragments into many cloudlets (“shattering”) or recoagulates back into one or a few larger clouds. The geometry (sheet / stream / sphere), the final overdensity χ_f, the metallicity, and the presence of a UV background all change the outcome — these movies show those effects.

Schematic of the implosion / explosion / shatter-or-coagulate sequence for sheets, streams, and spheres — Schematic of the implosion / explosion / shatter-or-coagulate sequence for sheets (left), streams (centre), and spheres (right). Reproduced from Fig. 21 of Yao et al. 2025.

Control parameters used in the movie titles

Geometry	Sheet, Stream, or Sphere. Determines whether converging flows are 1D, 2D, or 3D.
η = ρ_s,f/ρ_s,i	Cloud overdensity gain from initial to final equilibrium (set to 10 in all clips here).
χ_f = ρ_s,f/ρ_bg	Final cold-cloud overdensity relative to the hot background. This is the dominant parameter for whether the cloud shatters or coagulates.
StreamProj / SheetSlice	Visualisation type. StreamProj = density projection along the stream axis. SheetSlice = mid-plane slice through a planar sheet.

1. Geometrical effects: streams

The four clips below isolate the role of the final overdensity χ_f for cylindrical (stream) geometry at fixed η = 10. Increasing χ_f moves the final state from quiet recoagulation toward fully developed shattering, with a borderline regime in between. These movies correspond to the stream rows of Table 1 in the paper (η = 10, χ_f ∈ {100, 200, 400, 1000}).

StreamProj — η = 10, χ_f = 100

Stream geometry, η = 10, χ_f = 100 Coagulation

Lowest overdensity in the stream sweep. The cloud implodes, rebounds, and recoagulates into a single coherent stream.

StreamProj — η = 10, χ_f = 200

Stream geometry, η = 10, χ_f = 200 Borderline

Intermediate overdensity. Some transient cloudlets appear during the explosion phase and then recoagulate, while some remain long-lived — the borderline shattering regime.

StreamProj — η = 10, χ_f = 400

Stream geometry, η = 10, χ_f = 400 Shatter

Above the shattering threshold for streams. The post-rebound cold gas fragments into many small cloudlets that persist on long timescales.

StreamProj — η = 10, χ_f = 1000

Stream geometry, η = 10, χ_f = 1000 Shatter

Strongest stream case shown. Vigorous shattering produces a long-lived population of small cloudlets distributed along the stream axis.

2. Geometrical effects: sheets

For comparison with the stream sweep above, the sheet geometry case below shows the implosion/explosion sequence in 1D-converging (planar) geometry. Sheets generically sit in the “fast coagulation” regime in this paper, so even at χ_f values that shatter streams, the cold sheet recoagulates rapidly.

SheetSlice — η = 10, χ_f = 200

Sheet geometry, η = 10, χ_f = 200 Coagulation

Mid-plane slice through a planar sheet at the same χ_f = 200 as the borderline stream case. The sheet implodes, develops thin cooling layers, and recoagulates without producing long-lived cloudlets — illustrating that geometry alone shifts the outcome.

Notes

The full simulation suite explored in the paper covers all three geometries, six values of χ_f, multiple metallicities (0.03 – 1.0 Z_⊙), with and without a UV background, and a range of resolutions (see Table 1 of the paper). Only a subset of those runs are uploaded as movies here; further visualisations may be added to the playlist over time.

Sheets and streams are run with outflow boundary conditions; the analysis (clump finder, mass / size statistics, coagulation criterion, application to cold streams penetrating virial shocks) is reported in the paper.