Last updated August 25, 2025

Collision and Cannibalism: How Large Galaxies Absorb Smaller Ones

Mechanisms of galactic cannibalism

Galactic cannibalism is an astrophysical process by which a large galaxy attracts and absorbs smaller galaxies under the effect of gravity. This phenomenon contributes to the growth of giant galaxies and the formation of extended stellar halos.

Gravitational tidal forces deform and stretch the smaller galaxy, leading to the gradual loss of its stars and gas. Kinetic energy is dissipated, and the materials are progressively integrated into the dominant galaxy, creating stellar streams and matter currents.

This mechanism of cosmic hierarchy, predicted by the ΛCDM model, explains how the massive galaxies we observe today were formed and evolved through successive mergers with satellite galaxies. Our own Milky Way perfectly illustrates this process, having absorbed several dwarf galaxies throughout its history.

Accretion and assimilation processes

The accretion of satellite galaxies occurs through several mechanisms. First, tidal stripping tears stars and dark matter from the satellite galaxy, forming stellar streams and halos. Then, dynamical friction slows down the satellite galaxy, causing it to spiral toward the center of the primary galaxy. Finally, complete assimilation occurs when the satellite is disrupted and its constituents are incorporated into the host galaxy.

N.B.: Tidal stripping is an astrophysical process where stars and gas from a satellite galaxy are torn away by the gravitational tidal forces exerted by a more massive host galaxy. These forces can stretch the satellite galaxy and form stellar streams or matter currents observable around the dominant galaxy. This mechanism contributes to the formation of extended stellar halos and the morphological evolution of galaxies.

Cosmic consequences

This process alters the morphology, density profile, and chemical composition of the host galaxy. Observations of massive elliptical galaxies often show signatures of past mergers, such as rings, arcs, and diffuse matter flows.

Notable examples of galactic cannibalism and mergers in the local Universe
Galactic system	Type of interaction	Timescale	Observational evidence	Relative mass
Milky Way - Sagittarius Dwarf Galaxy	Ongoing accretion	2-4 billion years	Tidal streams, stellar overdensity	1:100
Milky Way - Canis Major Dwarf Galaxy	Recent accretion	1-2 billion years	Overdensity of M-giant stars	1:300
Milky Way - Enceladus Galaxy (Gaia-Enceladus)	Major past merger	8-10 billion years	Distinct stellar population in the halo	1:4
Andromeda Galaxy (M31) - M32	Advanced absorption	Past merger	Dynamical disturbances, thick disk	1:50
Andromeda Galaxy - Triangulum Galaxy (M33)	Future interaction	Collision expected in 2-4 billion years	Deformation of M33's outer arm	1:10
Sombrero Galaxy (M104)	Major merger	1-2 billion years	Extended halos, stellar shells	1:3 (merger)
Virgo Cluster - Elliptical galaxies	Cluster cannibalism	Ongoing	Multiple nucleus galaxies, tidal streams	Variable
NGC 4676 (The Mice)	Ongoing collision	150-200 million years	Extended tidal tails	1:1
Antennas (NGC 4038/4039)	Advanced merger	100-200 million years	Double tail, starburst	1:1.3
NGC 520	Intermediate merger	300 million years	Disturbed disk, matter bridge	1:1.5
NGC 3256	Starburst galaxy merger	400 million years	Tidal tail, double nuclei	1:1
NGC 1275 (Perseus Galaxy)	Active cannibalism with multiple galaxies	Ongoing	Filament systems, multiple nuclei	1:10 to 1:100
Cigar Galaxy (M82)	Interaction with M81	100 million years	Starburst, HI emission	1:2
NGC 7727	Advanced merger	1 billion years	Multiple shells, stellar streams	1:2
ESO 593-8 (Tinker Bell Triplet / IRAS 19115-2124)	Ongoing triple merger	Ongoing (active phase)	Three galactic nuclei, multiple tidal tails	1:1.2:2.5