Dark galaxy

A galaxy-like system dominated by dark matter and gas, with few or no stars and therefore little visible light. Its existence is inferred through effects such as gas emission or gravitational influence.

Dark matter halo: A gravitationally bound structure dominated by dark matter that can host gas, stars, and galaxies. Dark galaxies are proposed to occupy halos whose gas has not formed many stars.
HI 1225+01: An extended cloud of neutral hydrogen containing two concentrations, one associated with a dwarf galaxy. Its apparently starless component was proposed as a dark-galaxy candidate.
21-centimeter line: Radio emission from the hyperfine transition of neutral atomic hydrogen at a wavelength of about 21 centimeters. It can reveal gas in systems too faint to detect through starlight.
Ultra-diffuse galaxy: A galaxy with a large physical extent but exceptionally low surface brightness. It has stars, whereas a dark galaxy is defined by having very few or none.
Cosmic reionization: The era when radiation from early sources ionized most hydrogen in the intergalactic medium. Reionization could heat gas and suppress star formation in low-mass halos.
VIRGOHI21: A large neutral-hydrogen cloud in the Virgo Cluster once proposed as a dark galaxy. Its interpretation as a rotating, starless galaxy was challenged by later observations.
Neutral hydrogen cloud: A region of interstellar or intergalactic gas whose hydrogen atoms are mostly un-ionized. Dark-galaxy candidates are often first identified as unusually isolated hydrogen clouds.
Dwarf galaxy: A small galaxy with relatively few stars compared with large spiral and elliptical galaxies. Even faint dwarf galaxies contain stars that distinguish them from starless candidates.
Virial temperature: The characteristic temperature reached by gas as it falls into a gravitationally bound halo. It helps determine whether halo gas can cool enough to form stars.
RELHIC: A proposed population of gas-rich, starless halos confined by the pressure of the surrounding intergalactic medium. These systems offer a specific theoretical route to dark galaxies.

Linked from 1 page