Giant galaxies: collisions, mergers and absorption

Galaxy merger — Cosmic dance and tidal forces. The majestic spiral galaxy NGC 1532 (seen edge-on) undergoes the gravitational influence of its small neighbor, the lenticular galaxy NGC 1531, visible just above its central bulge.
Image source: ESO/IDA/Danish 1.5 m/R.Gendler and J.-E. Ovaldsen (new window)

Scientific abstract

The Astronoo article presents galactic cannibalism, a process by which a massive galaxy absorbs smaller galaxies under the effect of gravity. Observations from large telescopes and systematic surveys reveal stellar streams, tidal tails and extended halos, signatures of past or ongoing mergers. Dynamical friction and tidal stripping gradually tear stars and gas from satellites, which spiral toward the center of the host galaxy. The ΛCDM model predicts this hierarchical growth, confirmed by the history of the Milky Way and Andromeda, which have absorbed many dwarf galaxies.

How do collisions and absorption of galaxies transform certain galaxies into giants?

This article is devoted to galactic cannibalism, the process by which a massive galaxy attracts, distorts and then absorbs satellite galaxies under the effect of gravity. But how do these collisions and absorptions transform certain galaxies into giants? The answer lies in the cosmic hierarchy: the most massive galaxies were built by successive mergers, each absorption bringing stars, gas and dark matter. Gravitational tidal forces stretch the satellite galaxy, which gradually loses its stars in the form of stellar streams, while dynamical friction makes it spiral toward the center of the host galaxy. This phenomenon modifies the morphology, density profile and chemical composition of the dominant galaxy, and leaves observable signatures such as extended halos, stellar shells or multiple nuclei. The ΛCDM model predicts this hierarchical growth, confirmed by the history of the Milky Way, which has absorbed several dwarf galaxies, and by that of Andromeda and its satellites. The article details the mechanisms involved, the observational evidence and the most notable examples of these galactic mergers.

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Mechanisms of galactic cannibalism

A process of gravitational growth

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 to the formation of extended stellar halos.

Deformation by tidal forces

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

Cosmic hierarchy and the ΛCDM model

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

Accretion and assimilation process

Three stages of accretion

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 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 in which the stars and gas of a satellite galaxy are torn away under the effect of gravitational tidal forces exerted by a more massive host galaxy. These forces can stretch the satellite galaxy and form stellar streams or matter flows observable around the dominant galaxy. This mechanism contributes to the formation of extended stellar halos and to the morphological evolution of galaxies.

Cosmic consequences

Morphological and chemical modifications

This process modifies 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

The table below presents several galactic systems in which cannibalism or merger interactions have been identified, with their main observational characteristics.

Notable examples of galactic cannibalism and mergers in the local Universe
Galaxy 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)	Past major merger	8-10 billion years	Distinct stellar population in the halo	1:4
Andromeda Galaxy (M31) - M32	Advanced absorption	Past merger	Dynamical perturbations, thick disk	1:50
Andromeda Galaxy - Triangulum Galaxy (M33)	Future interaction	Collision predicted in 2-4 G 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	Perturbed 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 several 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

References

FAQ: everything you need to know about galactic cannibalism

What is galactic cannibalism?

Galactic cannibalism is the process by which a massive galaxy absorbs one or more smaller galaxies under the effect of gravity. This phenomenon, predicted by the ΛCDM model, contributes to the growth of giant galaxies and to the formation of extended stellar halos. It manifests itself through observable signatures such as stellar streams, tidal tails or multiple nuclei.

How does a galaxy absorb another galaxy?

Absorption takes place in several stages. First, gravitational tidal forces stretch the satellite galaxy and tear away its stars and gas (tidal stripping). Then, dynamical friction exerted by the dark matter halo of the host galaxy slows the satellite, which spirals toward the center. Finally, the satellite is completely disrupted and its constituents are incorporated into the dominant galaxy, modifying its morphology and chemical composition.

Does the Milky Way absorb other galaxies?

Yes. The Milky Way is currently engaged in the absorption of several dwarf galaxies, notably the Sagittarius Dwarf Galaxy and the Canis Major Dwarf Galaxy, whose stellar streams are observed in the galactic halo. It also experienced a major merger about 8 to 10 billion years ago with Gaia-Enceladus, whose distinct stellar population was identified by the Gaia mission.

What happens when two galaxies of comparable size collide?

When two galaxies of similar mass meet, the process is called a major merger. Tidal forces strongly deform both galaxies and trigger intense bursts of star formation. After several hundred million years, the two systems merge into a single galaxy, often elliptical, whose morphology and kinematics retain the trace of the collision.

Do stars collide during a galactic merger?

No, direct collisions between stars are extremely rare, because the distances separating them are gigantic compared to their size. On the other hand, interstellar gas clouds frequently collide, which produces shock waves and stimulates the formation of new stars. It is therefore the stellar orbits, and not the stars themselves, that are deeply perturbed.

Will the Milky Way merge with Andromeda?

Yes. The Andromeda Galaxy (M31) is approaching the Milky Way at about 110 kilometers per second. Simulations predict a first encounter in about 4 billion years, followed by a complete merger forming a large elliptical galaxy, sometimes nicknamed "Milkomeda". The stars themselves will not collide, but the solar system could be displaced to another region of the resulting galaxy.