Glossary

Comoving coordinates: Coordinates that expand together with the Universe, so that objects moving with the cosmic expansion keep fixed comoving positions. Comoving distances are related to physical (proper) distances by the scale factor.

Core: The innermost particles of a halo, tracked through the remainder of the simulation to follow the evolution of substructure. Halo and core assembly histories provide the structure-formation foundation for the Diffsky synthetic galaxy catalogs.

Friends-of-Friends (FoF): A halo-finding algorithm that links particles separated by less than a set linking length; a halo is a group of particles connected by such links. See Basics of Cosmological Simulations for details.

Gravity-Only Simulation: A simulation in which matter interacts only through gravity. Its particles represent the total matter content of the Universe, so it forms halos and large-scale structure but no stars or galaxies.

Halo: An overdense clump of matter formed through gravitational collapse. Halos host galaxies and are the primary objects in the portal's catalogs.

HAvoCC: The HACC Analysis and Validation to Observational Constraints Code, the framework behind the portal's prepackaged analysis tasks. Module-level definitions of the computed quantities are given in the HAvoCC documentation.

HEALPix: An equal-area pixelization of the sphere used for the portal's full-sky lightcone maps. Its resolution parameter nside sets the number of pixels (12 nside²).

Hydrodynamic Simulation: A simulation that models baryonic physics — gas dynamics, star formation, and feedback — in addition to gravity, allowing it to form galaxies.

Lightcone: The collection of objects as they would be observed from a fixed vantage point, with each object recorded at the moment its light would reach the observer. See Lightcone data.

$M_{200c}M\_\{200c\}$ and $r_{200c}r\_\{200c\}$: A standard spherical-overdensity mass and radius. $r_{200c}r\_\{200c\}$ is the radius within which the mean enclosed density equals 200 times the critical density of the Universe, and $M_{200c}M\_\{200c\}$ is the mass enclosed within that radius. Analogous quantities are defined for other thresholds, such as $M_{500c}M\_\{500c\}$, and are almost always given relative to the critical density.

Particle: The smallest unit of mass in a simulation — a mass element rather than a physical particle. Hydrodynamic simulations contain several particle species (dark matter, gas, stars, black holes), and particle masses differ among simulations; in hydrodynamic simulations the masses of gas, star, and black-hole particles can also change during a run.

Redshift and scale factor: Two equivalent ways of labeling cosmic time. The scale factor $aa$ measures the expansion of the Universe, with $a=1a\; =\; 1$ today, and redshift is $z=1\mathrm{/}a-1z\; =\; 1/a\; -\; 1$. Simulation outputs are labeled by step number; the dataset pages list the redshift corresponding to each step.

Snapshot: A complete simulation output at a fixed time, identified by its step number. Snapshot queries select data from a single snapshot, in contrast to lightcone queries.

Spherical Overdensity (SOD): A halo measurement made by growing a sphere from the halo center until the mean enclosed density falls below a threshold, typically a multiple of the critical density. See Basics of Cosmological Simulations for details.

Subgrid Model: A parameterized prescription for astrophysical processes — star formation, feedback, cooling — that act below the resolution of the simulation and are calibrated to match observations.

Unit conventions: Raw HACC outputs use comoving coordinates and include factors of the reduced Hubble parameter $hh$: masses, for example, are given in $M_{\odot }\mathrm{/}hM\_\backslash odot/h$. Data returned by the portal, when opened with the OpenCosmo Toolkit, defaults to the "comoving" convention, in which factors of $hh$ are absorbed into the values; other conventions are available. See Reading Data.