PICO-Cluster

The AREPO Code

The PICO-Cluster simulations were carried out with the AREPO code (AREPO-2 version), a moving-mesh magnetohydrodynamics (MHD) code evolved from the MillenniumTNG project. Collisionless components (dark matter, stars, SMBHs) are modelled as discrete particles with gravitational forces computed using a TreePM approach. The ICM gas is represented on an unstructured Voronoi mesh that follows the gas motion, providing the benefits of Lagrangian schemes. AREPO-2 features advances in memory management, load-balancing efficiency, and the Subfind-HBT algorithm for efficient merger tree construction.

MHD is evolved using a second-order finite-volume Godunov scheme with the HLLD Riemann solver. Divergence errors in the magnetic field are controlled with the Powell 8-wave scheme. Simulations are initialized with a uniform comoving magnetic seed field of 10⁻¹⁴ G at z = 127.

IllustrisTNG Galaxy Formation Model

The baseline physics incorporates the full IllustrisTNG galaxy formation model, including:

• Star formation following the Kennicutt-Schmidt relation with a Chabrier IMF
• Kinetic stellar feedback winds decoupled from hydrodynamics
• SMBH seeding, growth via Bondi-Hoyle accretion, and two-mode AGN feedback
• Gas cooling and heating including primordial and metal-line processes
• A spatially uniform redshift-dependent UV ionizing background

The IllustrisTNG model was calibrated on 25 h⁻¹Mpc volumes; PICO-Clusters tests it in the extreme cluster environment at unprecedented resolution.

CosmoZoomIC

Zoom-in initial conditions were created with the new CosmoZoomIC code (Puchwein et al., in prep.). The code uses a hierarchical tree structure to concentrate resolution in the Lagrangian region of each cluster and its environment out to 5 R₂₀₀, allowing the high-resolution zone to trace the arbitrarily shaped Lagrangian volume, with a gradual transition to the coarse background grid.

Two key techniques prevent contamination by low-resolution particles:

• Filling "holes" in the Lagrangian region by iterative neighbour-flagging
• Adding one additional layer of high-resolution grid points around the flagged region

All clusters are confirmed free of contamination within at least 2.74 R₂₀₀ at all simulation outputs, with the typical minimum contamination radius exceeding 4 R₂₀₀.

Left: minimum contamination radius vs. cluster mass for all simulations. Right: density distributions of high- and low-resolution particles at z=0. View full size →

Parent Box & Halo Selection

Clusters were selected from a 1 h⁻¹Gpc non-radiative MHD parent simulation with 1024³ DM particles (Planck 2018 cosmology). All 272 halos above 10¹⁵ M_☉ were identified at z = 0.

The 24 selected clusters have a flat distribution in log(M₂₀₀) between 10^14.9 and 10^15.4 M_☉, with the five most massive halos always included. This mass range extends well beyond the most massive clusters in TNG100 (3.8×10¹⁴ M_☉) and TNG50 (1.8×10¹⁴ M_☉).

The parent simulation showing the cosmic web (left), a string of clusters (lower right), and the Z24 zoom-in re-simulation of halo 4 (upper right). View full size →