Galactic tide

A galactic tide is a specific tidal force felt by objects within a galaxy's gravitational field. This force is not determined by the total strength of gravity alone. Instead, it depends on the gradient of the gravitational field. A gradient is the rate at which the strength of gravity changes over a certain distance. Because of this, tidal effects are usually limited to the immediate surroundings of a galaxy. These forces can reshape entire galaxies, strip material from smaller neighbors, and even influence the movement of icy objects in our own Solar System.

When two large galaxies pass near each other or undergo a collision, they experience massive tidal forces. These galaxies rarely collide head-on. Instead, the tidal forces distort each galaxy along an axis. This axis points roughly toward and away from the other galaxy, which is known as the perturber. As the galaxies briefly orbit one another, the distorted regions are pulled away from the main galactic bodies. The galaxy's own differential rotation then shears these regions. This process flings the material into intergalactic space, creating long, curved structures called tidal tails.

The appearance of these structures depends on the mass of the interacting galaxies. If the perturbed galaxy is equal to or smaller than its partner, a large tidal tail forms. However, if one galaxy is significantly more massive than the other, the results look different. The trailing arm remains relatively minor. In this case, the leading arm becomes more prominent and is often called a tidal bridge. These bridges can be difficult to see. They might be absorbed by a merging galaxy or obscured if one galaxy is in the foreground. In rare cases, a tidal loop can form where a tail joins its parent galaxy at both ends.

Satellite galaxies are also highly susceptible to these forces because they exist in the immediate vicinity of large galaxies. A large host galaxy can exert enough force to cause tidal stripping. This is a process where stars and gas are torn from the extremities of a satellite galaxy. For example, the dwarf galaxy M32 is a satellite of the Andromeda Galaxy. It may have lost its spiral arms due to tidal stripping. The gravitational pull can also knead and compress the interstellar gas clouds inside a small satellite. This compression can induce high rates of star formation within the remaining core.

The scale of the satellite galaxy also changes how the debris looks. If a satellite is very small, the tidal debris tails are likely to be symmetric. These tails follow a similar orbit and trace the path of the satellite. However, if a satellite is larger than one ten-thousandth the mass of its host, its own gravity may interfere. This can break the symmetry and accelerate the tails in different directions. Over many orbits, a dwarf satellite may be completely disrupted. This can result in a tidal stream of stars and gas that wraps around the larger host galaxy. Some scientists suggest that the extended discs of gas around galaxies like Andromeda are actually the results of such complete disruptions.

Tidal effects also reach into the very edges of our own Solar System. While a star's gravity usually dominates its own local system, that gravity becomes weak at great distances. In these outer reaches, the galactic tide can become significant. A major area of interest is the Oort cloud, a theoretical shell of icy objects surrounding the Solar System. The Oort cloud may be over a light-year in radius. Across such a massive distance, the gradient of the Milky Way's gravitational field plays a noticeable role.

The galactic tide can physically deform the Oort cloud. It can stretch the cloud toward the galactic center while compressing it along the other two axes. This is similar to how the Moon affects Earth's oceans. Because the Sun's gravity is so weak at these distances, these small galactic perturbations can dislodge planetesimals. These objects are composed of a mixture of rock and ice. When they are nudged from their orbits and move toward the inner Solar System, solar radiation turns them into comets. It is estimated that up to 90% of all comets originating from an Oort cloud may be the result of the galactic tide.