Glossary

Particle: A single unit of mass in the simulation. This can be of several types (dark matter, stars, gas) depending on the simulation. In general these are of order ${10}^8{M}_{\odot }10^8\; M\_\{\backslash odot\}$.

Gravity-Only Simulation: A simulation that only models the effect of gravity. Gravity-Only simulations are initialized such that the average density in the box is equal to the critical density of the universe.

Hydrodynamic Simulation: A simulation that models the effects of hydrodynamics (e.g. thermal interactions of gas) in order to simulate galaxy formation.

Subgrid Model: The effects of hydrodynamics are relevant on scales smaller than the spatial resolution of the simulation. The subgrid model is responsible for handling these effects on these scales.

Friends-of-Friends (FoF): When searching for halos in a simulation we use a "friends of friends" algorithm. We first select a particle and find any other particles within a set distance of the chose particles. These are known as the "friends" of the original particle. We repeat this process on any "friends" that are found to find "friends of friends." We continue to repeat the process until we are left with particles whos only "friends" are particles that have already been labeled as part of the halo.

Spherical Overdensity (SOD): Another way of measuring halo properties is with a "Spherical Overdensity (SOD) model. We center a sphere on a density peak found with the FoF algorithm. We then gradually increase the size of the sphere until the average density within it drops below a certain threshold value. For older, relaxed halos the properties measured with the SOD algorithm will be similar to the properties measured with the FoF algorithm. For younger halos, or halos undergoing a merjer, these two methods can return substantially different results.

$M_{200c}M\_\{200c\}$ and $r_{200c}r\_\{200c\}$: The SOD algorithm requires a threshold value to determine when to cutoff the halo. One standard cutoff is $200{\rho }_{c}200\backslash rho\_c$, where ${\rho }_c\backslash rho\_c$ is the critical density of the universe. $r_{200c}r\_\{200c\}$ is the radius at which the average density enclosed by the sphere drops below this value. $M_{200c}M\_\{200c\}$ is the total mass enclosed inside the sphere with this radius. Other values besides 200 can also be measured, but are almost always given in terms of the critical density of the universe.