Basics of Cosmological Simulations
This page introduces the concepts needed to interpret the data products available on the portal: how the simulations are run, what the particles represent, and how halos and galaxies are identified.
N-body Simulations
Cosmological N-body simulations follow the formation of structure by evolving a large collection of particles under their mutual gravitational attraction, as described by the Vlasov-Poisson equations. The simulations on this portal were run with the Hardware/Hybrid Accelerated Cosmology Code (HACC), which solves this system efficiently at cosmological scales by splitting the force computation into a long-range part, handled by a spectral solver built on the SWFFT parallel FFT library, and a short-range part, handled by tree- and particle-based solvers optimized for a range of supercomputer architectures.
Gravity-Only vs. Hydrodynamic Simulations
In a gravity-only simulation, matter interacts exclusively through gravity. Such simulations are sometimes called "dark-matter-only", but that label is imprecise: the particles represent the total matter content of the Universe, dark and baryonic alike, evolved as if gravity were the only force acting on it. A gravity-only universe forms no stars or galaxies, but it does form halos and large-scale structure.
Hydrodynamic simulations additionally model the baryonic component: gas dynamics, star formation, and the associated astrophysical processes that produce galaxies.
Gravity-only simulations remain essential for several reasons. They require roughly an order of magnitude less computing than hydrodynamic simulations of the same size, which permits larger volumes and higher mass resolution. Because they contain no subgrid astrophysics, their predictions for halo populations and matter clustering are free of the uncertainties associated with subgrid-model choices. They also provide the structural foundation for synthetic galaxy catalogs, and, when paired with a matching hydrodynamic run, a baseline against which baryonic effects can be measured directly.
Particles
The smallest unit of mass in a cosmological simulation is the particle — not a particle in the physical sense, but a mass element whose mass is set by the mass resolution of the simulation and therefore differs from simulation to simulation. Gravity-only simulations contain a single particle species. Hydrodynamic simulations distinguish several: dark matter, gas, stars, and black holes. Dark-matter and gravity-only particle masses are fixed for the duration of a run, while gas, star, and black-hole particle masses evolve through processes such as star formation, stellar mass loss, and black-hole accretion, so particle masses can differ even within a single simulation.
Hydrodynamics Solver
HACC models hydrodynamics with a Lagrangian method, in which particles are treated as fluid elements. The specific scheme is Conservative Reproducing Kernel Smoothed Particle Hydrodynamics (CRK-SPH), a high-order SPH formulation that enforces conservation of mass, linear momentum, and energy while maintaining approximate conservation of angular momentum. Hydrodynamics requires much finer time resolution than gravity alone, so the fluid equations are advanced in a sub-cycling loop within each global timestep, executed on GPUs.
Subgrid Models
Many astrophysical processes act on scales far smaller, and time intervals far shorter, than a large cosmological simulation can resolve, making it impossible to simulate them from first principles. They are instead represented by subgrid models: approximate, parameterized prescriptions calibrated to match observations. Typical subgrid models cover star formation, metal enrichment, supernova and active galactic nucleus (AGN) feedback, and radiative and metal-line cooling. Because these processes demand very high time resolution, they are integrated with an adaptive scheme based on local conditions.
Halo Finding
Particles in a simulation collect into overdense structures, formed through gravitational collapse, known as halos. Locating halos and measuring their properties is called halo finding. The halo catalogs on this portal contain two families of halo measurements: friends-of-friends (FoF) and spherical overdensity (SOD).
Friends-of-Friends (FoF)
The FoF algorithm builds halos by linking particles. Starting from any particle, all particles within a set linking length are marked as its "friends"; the same marking is applied to each friend in turn, and the process repeats until no new particles are added. The resulting connected set of particles is an FoF halo, and its properties — total mass, bulk velocity, and so on — are measured from its member particles.
Because the algorithm treats all particles equivalently, halo catalogs for hydrodynamic simulations are constructed by running the FoF finder on the dark matter particles only, excluding the other species from the halo definition.
Spherical Overdensity (SOD)
SOD measurements start from the FoF halos. Centered on the minimum-potential point of an FoF halo, a sphere is grown until the mean density it encloses — the total mass of all particles inside the sphere divided by its volume — falls below a chosen threshold, conventionally expressed as a multiple of the critical density of the Universe. For a threshold of 200 times the critical density, the resulting radius and enclosed mass are denoted and . In the portal catalogs these correspond to the sod_halo_radius and sod_halo_mass columns. Depending on the catalog, measurements at other overdensities, such as , may also be provided.
In hydrodynamic simulations, SOD quantities include the particles of all species within the sphere, not only dark matter. This makes SOD masses the standard mass definition for hydrodynamic halo catalogs.
Galaxy Finding
Galaxies are identified from the star particles using DBSCAN, a density-based clustering algorithm that generalizes friends-of-friends: rather than linking any pair of particles within a set distance, it builds groups around star particles that have at least 10 neighbors within 50 kpc (in proper units). The center of each galaxy is its minimum-potential point, and galaxy properties are measured within a fixed 50 kpc proper-distance aperture, analogous to the SOD measurements for halos. The DBSCAN membership information is retained alongside the aperture-based properties.