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.