Did galaxies form before or after dark matter halos?

In the standard picture, dark matter halos collapse first because dark matter does not radiate energy. Ordinary gas then falls into these halos, cools, and forms stars, so the luminous galaxy assembles inside a pre-existing halo.

Why do galaxies stop forming stars?

Galaxies can run low on cold gas, and energetic feedback from supernovae and active galactic nuclei heats or expels gas, quenching star formation. Environment, such as residing in a cluster, can also strip away the gas supply.

Galaxy Formation and Evolution

Galaxy formation and evolution studies how galaxies emerge from primordial density fluctuations and grow through gas accretion, star formation, mergers, and feedback over cosmic time.

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Definition

Galaxy formation and evolution is the branch of astrophysics that explains how galaxies originate within growing dark matter halos and change over time through the inflow and cooling of gas, the formation of stars, the energetic feedback that regulates that growth, and mergers with other galaxies.

Scope

This area covers the hierarchical growth of structure from dark matter halos, the assembly of galaxies through mergers and interactions, the regulation of star formation by gas supply and feedback, and the observed evolution of the galaxy population from the early universe to the present.

Sub-topics

Core questions

How do galaxies form within the cosmic web of dark matter halos?
What roles do mergers and interactions play in shaping galaxies?
How is star formation regulated by gas supply and feedback?
How has the galaxy population evolved across cosmic time?

Key theories

Two-stage hierarchical formation: White and Rees proposed that dark matter halos form first by gravitational collapse and clustering, after which gas cools within them to form the luminous galaxies, setting the framework for modern theory.
Feedback-regulated growth: Energy from supernovae and active galactic nuclei heats and ejects gas, suppressing star formation and shaping the galaxy mass function within the cosmological model.
The cosmic star-formation history: The volume-averaged rate of star formation rose to a peak several billion years after the Big Bang and has since declined, a history that any successful model of galaxy evolution must reproduce.

Clinical relevance

Galaxy formation ties together cosmology, gas dynamics, star formation, and black hole growth into a single narrative of how the visible universe assembled, and it provides the theoretical backbone for interpreting deep surveys and next-generation telescopes.

History

The modern picture began with the 1978 White and Rees two-stage theory linking galaxies to dark matter halos. Numerical simulations and semi-analytic models matured through the 1990s and 2000s, and deep surveys mapped the cosmic star-formation history, together establishing feedback-regulated hierarchical formation as the standard framework.

Key figures

Simon White
Martin Rees
Piero Madau
Rachel Somerville

Seminal works

white1978
madau2014
mo2010

Frequently asked questions

Did galaxies form before or after dark matter halos?: In the standard picture, dark matter halos collapse first because dark matter does not radiate energy. Ordinary gas then falls into these halos, cools, and forms stars, so the luminous galaxy assembles inside a pre-existing halo.
Why do galaxies stop forming stars?: Galaxies can run low on cold gas, and energetic feedback from supernovae and active galactic nuclei heats or expels gas, quenching star formation. Environment, such as residing in a cluster, can also strip away the gas supply.