Galactic halo

A roughly spherical region surrounding a galaxy’s disk, containing stars, globular clusters, hot gas, and dark matter. Its components reveal how galaxies formed and how their mass is distributed.

Dark matter halo: A dark-matter-dominated structure surrounding a galaxy, inferred primarily from its gravitational effects. It supplies most of the halo’s mass and shapes the galaxy’s gravitational environment.
Virial theorem: A relation connecting the average kinetic and potential energies of a stable, gravitationally bound system. It links a halo’s gravitational binding to the motions of its stars and gas.
Galactic cannibalism: The growth of a galaxy through the gravitational capture and merger of smaller galaxies. Disrupted satellites deposit stars and clusters into the surrounding halo.
Galaxy rotation curve: A plot of orbital speed in a galaxy against distance from its center. Outer rotation speeds constrain the mass enclosed by the halo.
Cold dark matter: A dark-matter model in which the particles move slowly compared with the speed of light during structure formation. Its predictions for small halo structures are tested against observed galaxies and satellites.
Stellar halo: A diffuse population of stars surrounding a galaxy, often assembled from disrupted smaller galaxies. Its stars preserve evidence of the halo’s formation history.
Hierarchical structure formation: The cosmological process in which small gravitationally bound structures merge into larger ones. Galactic halos grow through mergers and accretion of smaller systems.
Stellar stream: An elongated trail of stars stripped from a disrupted star cluster or dwarf galaxy. Streams mark recent accretion events still unfolding in a halo.
Gravitational lensing: The bending of light by gravity, which can reveal the mass of foreground objects. Lensing maps halo mass even where it emits little or no light.
Missing satellites problem: The mismatch between predicted numbers of small dark-matter subhalos and observed dwarf galaxies. It asks why many predicted halo substructures lack visible galaxies.

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