A galaxy merger combines two or more galaxies into a remnant. Their orbital angular momentum and energy are redistributed into internal stellar and halo motions, escaping matter, and, when present, dissipative gas. A bound pair produced by tidal capture of galaxies need not coalesce immediately.
Tidal capture converts sufficient relative orbital energy into internal tidal heating to leave two initially unbound galaxies on a bound orbit. For identical spherical galaxies of mass , a distant rapid flyby of impact parameter and relative speed gives the approximate capture condition . This requires the impulse approximation and weak gravitational focusing; it is not a general criterion for subsequent coalescence.
A dry galaxy merger joins galaxies with little cold gas, so dissipation and merger-triggered star formation are weak. Conservation of energy followed by re-virialization can make repeated dry minor mergers increase a galaxy's radius faster than its mass.
A minor galaxy merger has a small satellite-to-host mass ratio. Its stars are commonly deposited at large host radii after tidal stripping and Chandrasekhar dynamical friction.
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A galaxy merger is an event in which two or more galaxies collide and interact with each other due to their gravitational forces. This phenomenon can lead to a variety of outcomes depending on factors such as the sizes, masses, and types of the merging galaxies, as well as their relative velocities. During a galaxy merger, the structures and dynamics of the galaxies involved can be significantly altered.