Illustris

The original Illustris run

Illustris was one of the first cosmological simulations large and detailed enough to produce a realistic mix of galaxy shapes in a single run: elliptical galaxies, spiral discs, and the thin filaments of the cosmic web that connect them. Its first results were published in 2014, and the full data were made public the following year.

Unlike earlier simulations, Illustris followed gas with a moving-mesh code, so flows and shocks were captured without the limitations of a fixed grid. That mattered for modelling the gas that forms stars and feeds black holes.

One cube, a universe of galaxies

The main run simulates a cubic volume about 106.5 megaparsecs on a side, with billions of resolution elements tracking dark matter and ordinary gas. The simulation advances step by step from shortly after the Big Bang to the present day, letting astronomers follow individual haloes and the galaxies that grow inside them.

Lower-resolution runs and smaller test volumes sit alongside the flagship cube, providing a range of scales for checking that results do not depend on resolution or box size.

What it showed

• Galaxy colours: the observed split between blue, star-forming galaxies and red, quenched ones.
• Galaxy shapes: the emergence of both elliptical and disc galaxies from the same initial conditions.
• The cosmic web: filaments, sheets and voids in the large-scale matter distribution.
• Black-hole feedback: supermassive black holes that heat gas and help shut down star formation.

From Illustris to TNG

Illustris demonstrated the approach, and its successor IllustrisTNG rebuilt it with magnetic fields, improved feedback and three matched volumes. The original run remains a landmark, and its data are still available at the Illustris project site.