Words used here, in plain language

§1Terms

Anchor

The observed curve that the Explorer measures offsets against by default for a relation. You can change it. An offset from the anchor is a verdict only where the definitions agree.

Aperture

The region around a galaxy inside which stars are counted. A larger aperture gives a larger stellar mass, so two curves made with different apertures are not measuring the same thing.

Black hole occupation

The fraction of galaxies in a bin of stellar mass that hold a black hole at all. In these catalogues a galaxy is a Subfind subhalo with at least one star particle. See seeding.

Black hole feedback

Energy released as gas falls onto the supermassive black hole at a galaxy's centre. It can heat gas or blow it out, and it is one of the two physics ingredients the dials page lets you turn.

CAMELS

A set of thousands of small simulated universes (25 Mpc/h boxes) run with different physics settings, for two codes, IllustrisTNG and SIMBA. It is the experiment behind the dials page.

Comparable

A tick on a curve in Check your own run: the definitions you declared agree with the curve's, so an offset against it is a fair comparison. A tilde means comparable with caveats; a cross means not comparable.

Context only

A curve shown for context but never used to rank or to compute offsets, usually because it was read from a figure or its definitions are incomplete.

Dex

A factor of ten, the unit of logarithms: 0.3 dex is a factor of two, 0.1 dex is about 26%.

Dial

One of four numbers that scale the strength or speed of supernova winds and black hole feedback in a simulation. The dials of the two codes are not the same physical thing.

Digitized figure

A curve read from a published plot because no table exists. Marked with a dotted line. Its accuracy is limited by the figure.

Emulator

A fast mathematical stand-in fitted to many simulations. It predicts what a simulation would give for any dial setting without running it. The dials page uses polynomial emulators fitted to 1,000 simulations per code.

Epoch, redshift (z)

How long ago the light left a galaxy. z = 0 is today; z = 2 is about 10 billion years ago.

Fiducial

The default setting of the dials, the one the simulation's authors chose.

Gap

On the dials page: how far apart the two codes' curves are for a property, in units of the scatter between two runs of the same code that differ only in their random seed. Below 1 they agree as well as two seeds of one code would. Elsewhere called a noise unit.

IMF (initial mass function)

How many stars of each mass form. Different IMFs give different stellar masses and star formation rates for the same light, so curves with different IMFs are not directly comparable.

Latin hypercube

A way of spreading simulation settings evenly across the whole range of every dial, so that each simulation differs from the others in all dials at once. CAMELS has 1,000 per code.

Metallicity

The share of a galaxy's gas or stars made of elements heavier than helium. Measures how much enrichment from earlier generations of stars has happened.

Noise unit

See Gap.

Quenched fraction

The share of galaxies that have stopped forming stars.

Rankable

The word the data files use for whether a curve’s definitions and sampling allow it to be used in a quantitative comparison. The pages say “fair to compare”.

Relation (scaling relation)

A curve linking two properties of galaxies, such as stellar mass and the number of galaxies of that mass.

Removable

A difference that retuning can close: with the best dial settings of each code, the gap falls to one noise unit or less.

Seeding (black hole)

The step in a simulation where a black hole is first put in, by a rule: when a halo or a galaxy passes a size threshold, the code places a black hole of a set mass. The two codes use different rules; see how the two codes plant black holes. Not to be confused with the random seed.

Seed scatter

The difference between two runs of the same simulation that differ only in their random starting conditions. It sets the scale below which two codes cannot be told apart.

Star-forming main sequence

The tight relation between how many stars a star-forming galaxy has and how fast it is making new ones.

Stellar mass function

How many galaxies there are of each stellar mass, per unit volume.

Stellar-to-halo mass ratio

How much of a dark matter halo's mass has turned into stars.

Structural

A difference that stays above one noise unit at every dial setting searched. It means beyond the scatter between runs, not physically large: for gas metallicity it is about 0.07 dex.

Subfind

The program that finds galaxies (subhaloes) in a simulation’s particles. Both CAMELS codes are read with the same Subfind, so galaxies are defined alike.

Subgrid physics

Processes too small to simulate directly (supernovae, black hole growth) that the code represents by rules with adjustable strengths.

Supernova winds

Gas blown out of a galaxy by exploding stars. Their strength and speed are two of the four dials.

Tier

How a curve was obtained: official table, paper table, published fit, catalogue-derived, or digitized figure. The Explorer shows it as a badge on every curve.

Unresolved

A property whose gap sits near one noise unit, so the evidence does not say whether retuning can close it.