Halos

Halos are roughly spherical regions that envelop galaxies and galaxy clusters. The term is most often used to describe the dark matter halo that dominates the gravitational potential of a galaxy, extending well beyond the visible stellar disk or spheroid. In addition to dark matter, halos may contain a sparse population of old stars, globular clusters, and diffuse, hot gas.

Dark matter halos follow characteristic density profiles and set the scale for a galaxy’s formation and dynamics. The luminous halo, or stellar halo, consists mostly of old, metal-poor stars and tidal debris from disrupted satellites. The gaseous halo, often referred to as the circumgalactic or intragalactic medium, can be hot and ionized with temperatures around 10^6 to 10^7 kelvin; cooler gas may also be present in some halos.

Halos form through hierarchical structure formation: small halos merge and accrete to build larger halos, while their substructure—subhalos, streams, and satellites—persists as evidence for assembly history. Feedback processes from stars and black holes shape the baryonic content of halos and influence star formation rates within galaxies.

Observationally, halos are inferred from galaxy rotation curves that imply unseen mass, gravitational lensing, the dynamics of satellite systems, and X-ray emission from hot gas. The Milky Way and Andromeda provide detailed case studies; their halos contain inner and outer stellar components, and numerous tidal streams reveal ongoing accretion.

Halos are also studied around galaxy clusters, where the dominant dark matter halo extends over megaparsecs and the intracluster medium fills the space between galaxies. Understanding halos is central to theories of galaxy formation and the distribution of dark matter in the universe.