Tidal dwarf galaxy

A galaxy formed from gas and stars pulled from larger galaxies by tidal forces during an interaction or merger. Unlike most galaxies, it forms from material already enriched by earlier generations of stars.

Galaxy interaction: A close encounter between galaxies that alters their structure through gravity, gas dynamics, and sometimes a merger. Gravitational encounters pull out the long streams of material where tidal dwarfs can condense.
Dwarf galaxy: A galaxy substantially smaller and less luminous than a large spiral or elliptical galaxy. Tidal dwarfs belong to this broad class but arise through a distinct formation route.
Fritz Zwicky: A Swiss-American astronomer who proposed dark matter and studied galaxy interactions and compact stellar systems. His early work on interacting galaxies helped frame the possibility of galaxy formation from debris.
Metallicity: The abundance of elements heavier than helium in an astronomical object. Their enriched gas can reveal whether a dwarf formed from material previously processed inside a larger galaxy.
Dwarf spheroidal galaxy: A faint, gas-poor galaxy with a roughly spheroidal shape and little ongoing star formation. Long-term survival of tidal dwarfs may help explain whether some faint satellites share this class's properties.
Tidal tail: An elongated stream of stars and gas drawn from a galaxy by gravitational tides. Tidal dwarfs commonly form as dense condensations within these stripped streams.
Dark matter halo: A gravitationally bound distribution of dark matter surrounding a galaxy or galaxy cluster. Tidal dwarfs are expected to contain little of the dark matter that dominates typical dwarf halos.
Antennae Galaxies: A pair of colliding galaxies whose long tidal tails contain bright knots of gas and young stars. These tails provide a prominent nearby setting for identifying candidate tidal dwarf galaxies.
Baryonic Tully–Fisher relation: An empirical relation between a galaxy's total ordinary matter and its characteristic rotation speed. Tidal dwarfs test whether this relation also holds for systems with unusual formation histories.
Satellite galaxy: A galaxy gravitationally bound to and orbiting a more massive host galaxy. The survival and later orbits of tidal dwarfs bear on the origins of satellite populations.

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