White and Rees (1978): Galaxies Form Inside Dark Matter Halos

Simon White and Martin Rees proposed in 1978 that galaxies form in two stages: dark matter first clumps into large halos, then gas cools and sinks to their centers to form stars. The idea that every galaxy is the bright core of a dark halo still underpins galaxy formation theory.

The question

By the late 1970s there were growing hints that galaxies are surrounded by large amounts of unseen mass, and that galaxies cluster together on large scales. Two puzzles followed. Why are the luminous parts of galaxies small and concentrated if most of their mass is spread out in an extended halo? And why do galaxies come with characteristic masses, rather than in every possible size? White and Rees asked how a single process could build both the halos and the galaxies inside them.

What the paper found

• Galaxy formation happens in two stages. Dark matter halos assemble hierarchically, small clumps merging into bigger ones, driven only by gravity.
• Gas then cools, loses energy and condenses at the centers of the halos, forming the visible galaxies.
• This explains why the luminous parts of galaxies are small and concentrated compared with their extended dark halos.
• The characteristic masses of galaxies are set by how efficiently gas can cool: in very massive halos the gas stays too hot to collapse quickly.

How they did it

The paper is theoretical. White and Rees combined a model of how structure grows by gravity, with dark matter clumping and merging over time, with the physics of how hot gas radiates away its energy. Comparing the time gas needs to cool with the time it takes to fall inward, they worked out which halos can host a compact, star-forming galaxy and which cannot. The dark halos act as gravitational wells, and the galaxies are what happens to the gas that settles at the bottom.

Why it mattered

White and Rees set out the picture most astronomers still use. It anticipated the cold dark matter models of the 1980s, notably the 1984 cold dark matter paper by Blumenthal and colleagues, and it fits naturally with the merger-driven growth that Toomre and Toomre simulated in 1972. Later work on the shape of dark halos, such as the Navarro-Frenk-White density profile of 1997, built on the same framework.

What we know now

The two-stage idea is the conceptual basis of modern “semi-analytic” models and of the large hydrodynamical simulations that follow gas and dark matter together. The newest test comes from JWST: its surprisingly early, bright galaxies, such as the massive candidates reported by Labbé and colleagues in 2023, are probing how efficiently the gas-cooling step can work in the young universe. Our galaxies guide explains how galaxies formed.

Who wrote it

Simon D. M. White is now an emeritus director of the Max Planck Institute for Astrophysics in Garching, Germany, an institute widely regarded as a world leader in theoretical astrophysics. Martin Rees is based at the Institute of Astronomy at the University of Cambridge. Semantic Scholar lists about 3,000 citations as of October 2026, making it one of the most cited papers on galaxy formation.

Read the original

White, S. D. M. and Rees, M. J. (1978). “Core condensation in heavy halos: a two-stage theory for galaxy formation and clustering.” Monthly Notices of the Royal Astronomical Society 183, 341-358. doi:10.1093/mnras/183.3.341

Free to read: a scanned PDF on NASA ADS.