A '''galactic tide''' is a galactic collisions, the disruption of dwarf or satellite galaxies, and the Milky Way's tidal effect on the hypothesized Oort Cloud of our own solar system.
==Origin==
thumb|left|250px
thumb|left|250px|A body in proximity to a larger mass becomes stretched out by tidal forces
When one body (like the blue object in the diagrams at left) is in the gravitational field of a large mass (the yellow object), it becomes tidally distorted. 

Gravitational attraction increases with decreasing distance; the closer an object is to another, the more intensely it is affected by the other object's gravity. This is also true of the different ''parts'' of an object; thus the surface of an object is more strongly attracted to another object's gravity than its core. When the other object's gravity is particularly strong, this causes the smaller object's surface to pull away from the core, and the object to distend and flatten in the direction of the larger object. The large body feels a similar but far weaker distortion caused in the same way by the gravitational field of the small body. In technical terms, the equilibrium shape of the small body is the one that minimizes its gravitational potential energy. In empty space, this would be a sphere. However, in the proximity of the large body, the lowest potential energy shape is an ellipsoid stretched along the axis connecting the two bodies.

For example, the tides on Earth are caused by the distortion that the Moon and the Sun cause to the Earth's gravitational field. In this case, the Earth's rotation is slow enough that the Earth is able to reshape itself so that the distortions remain oriented approximately in the direction of the Moon and the Sun. From the point of view of a person on the surface, we pass over the long axes of the distortions approximately twice a day for each, at which points high tide are experienced. Since the relative positions of the Earth, Sun and Moon are constantly changing, the tidal effects reinforce or counteract each other to various degrees. (see Spring tide and Neap tide).

Galactic tides demonstrate the same processes on a far grander scale. Tidally interacting galaxies will be stretched towards each other. They may eventually flatten out and distend towards the galaxy's centre, or suffer perturbations to their orbits. Furthermore, if the galaxies are rapidly rotating, their sections may not be able to keep up with the distortion like the Earth, and long tails of stars and other highly distorted regions can be formed, as seen in the diagrams in this article.

==Effects on external galaxies==
 

=== Galaxy collisions ===
Tidal forces are dependent on the gradient of a gravitational field, rather than its strength, and so tidal effects are usually limited to the immediate surroundings of a galaxy. Two large galaxies undergoing collisions or passing nearby each other will be subjected to very large tidal forces, often producing the most visually striking demonstrations of galactic tides in action. 

Two interacting galaxies will not always collide head-on (if at all), and the tidal forces will distort each galaxy along an axis pointing roughly towards - and away from - its perturber. As the two galaxies briefly orbit each other, these distorted regions, pulled away from the main body of each galaxy, will be sheared by the galaxy's differential rotation and flung off into intergalactic space, forming '''tidal tails'''. Such tails are typically strongly curved; where a tail appears straight, it is probably being viewed edge-on. The stars and gas that comprise the tails will have been pulled from the easily distorted galactic discs (or other extremities) of one or both bodies, rather than the gravitationally bound galactic centres.<ref name="toomre72"></ref> Two very prominent examples of collisions producing tidal tails are the Mice Galaxies and the Antennae Galaxies.

thumb|300 px|The [[Mice Galaxies NGC 4676]]
Just as the Moon raises two water tides on opposite sides of the Earth, so a galactic tide produces two arms in its galactic companion. While a large tail is formed if the perturbed galaxy is equal to or less massive than its partner, if it is significantly more massive than the perturbing galaxy, then the trailing arm will be relatively minor, and the leading arm, sometimes called a '''bridge''', will be more prominent.<ref name="toomre72"/> Tidal bridges are typically harder to distinguish than tidal tails: in the first instance, the bridge may be absorbed by the passing galaxy or the resulting merged galaxy, making it visible for a shorter duration than a typical large tail. Secondly, if one of the two galaxies is in the foreground, then the second galaxy — and the bridge between them — may be partially obscured. Together, these effects can make it hard to see where one galaxy ends and the next begins. '''Tidal loops''', where a tail joins with its parent galaxy at both ends, are rarer still.<ref></ref>

=== Satellite interactions ===
thumb|left|300 px|The [[Andromeda Galaxy. Note its satellite galaxy M32 (top left), whose outer arms have been stripped away by Andromeda's tidal forces.]]
Because tidal effects are strongest in the immediate vicinity of a galaxy, satellite galaxies are particularly likely to be affected. Such an external force upon a satellite can produce ordered motions within it, leading to large-scale observable effects: the interior structure and motions of a dwarf satellite galaxy may be severely affected by a galactic tide, inducing rotation (as with the tides of the Earth's oceans) or an anomalous mass-to-luminosity ratio. <ref></ref> Satellite galaxies can also be subjected to the same tidal stripping that occurs in galactic collisions, where stars and gas are torn from the extremities of a galaxy, possibly to be absorbed by its companion. The dwarf galaxy Andromeda, may have lost its spiral arms to tidal stripping, while a high star formation rate in the remaining core may be the result of tidally-induced motions of the remaining molecular clouds<ref name="Bekkietal2001">{{cite journal
  | author=Bekki, Kenji; Couch, Warrick J.; Drinkwater, Michael J.; Gregg, Michael D.
  | title=A New Formation Model for M32: A Threshed Early-Type Spiral Galaxy?
  | journal=The Astrophysical Journal
  | year=2001
  | volume=557
  | pages=Issue 1, pp. L39–L42
  | url=http://adsabs.harvard.edu/abs/2001ApJ...557L..39B
  | doi=10.1086/323075 }}</ref> (Because tidal forces can knead and compress the interstellar gas clouds inside galaxies, they induce large amounts of star formation in small satellites. The process is somewhat similar to making something hotter by squeezing it).

The stripping mechanism is the same as between two comparable galaxies, although its comparatively weak gravitational field ensures that only the satellite, not the host galaxy, is affected. If the satellite is very small compared to the host, the tidal debris tails produced are likely to be symmetric, and follow a very similar orbit, effectively tracing the satellite's path.<ref></ref> However, if the satellite is reasonably large - typically over one ten thousandth the mass of its host - then the satellite's own gravity may affect the tails, breaking the symmetry and accelerating the tails in different directions. The resulting structure is dependent on both the mass and orbit of the satellite, and the mass and structure of the conjectured galactic halo around the host, and may provide a means of probing the potential of a galaxy such as the Milky Way.<ref></ref>

La '''marea galattica''' è una galassie nane o satelliti, nonché, in scala ridotta, gli effetti di marea sull'ipotizzata Nube di Oort del nostro Sistema Solare.

==Origine==
thumb|left|250px
thumb|left|250px|A body in proximity to a larger mass becomes stretched out by tidal forces

Quando un corpo (come l'oggetti blu nel diagramma a sinistra) si trova nel campo gravitazionale di una grande massa (l'oggetto giallo), viene distorto dalla forza di marea.

L'attrazione gravitazionale aumenta con il diminuire della distanza: più un oggetto è vicino ad un altro, maggiormente risente dell'influenza gravitazionale dell'altro oggetto. Quando un oggetto molto grande subisce l'influenza gravitazionale di un altro, la forza gravitazionale può variare considerevolmente da una parte all'altra dell'oggetto. Questo tende a distorcerne la forma, senza cambiarne il volume. Supponendo che l'oggetto fosse inizialmente una sfera, le forze di marea tenderanno a distorcerlo in un ellissoide, con l'asse maggiore allineato verso il corpo che produce la forza di gravità.

Le forze di marea seguono la legge dell'inverso del cubo. La forza di marea esatta in ogni punto è descritta dal tensore di Weyl.

Come sulla Terra la forza di marea lunare si avverte con l'aumento ciclico del livello del mare (marea), anche in scala galattica si avvertono le forze di marea; due galassie interagenti per marea tenderanno a distorcersi, allungandosi. Inoltre, se le galassie possiedono un rapido moto di rotazione, le loro sezioni potrebbero non essere in grado di reggere la sollecitazione, come diversamente succede con la Terra, e possono formarsi lunghe code di stelle e altre regioni altamente distorte, come si vede nel diagramma a fianco.

==Effetti sulle galassie==

===Collisione fra galassie===
Le forze mareali dipendono dal gradiente di campo gravitazionale, piuttosto che dalla loro forza, perciò gli effetti di marea sono di solito limitati agli immediati dintorni della galassia. Due grandi galassie che vanno incontro a collisione o si sfiorano a vicenda saranno soggette ad una grande forza di marea, i cui effetti sono ben evidenti nei telescopi.

Due galassie interagenti non sempre interagiscono in maniera diretta, e le forze di marea distorcono entrambe le galassie lungo un asse che punta grosso modo (e all'esterno) la causa della sue perturbazione. Come due galassie entrano in orbita stretta l'una attorno all'altra, queste regioni distorte, spinte via dal corpo principale della galassia, vengono strappate via dalla rotazione differenziale della galassia stessa e lanciate nello spazio, formando le ''code mareali''. Queste code sono di solito fortemente curvate; quando una coda appare rettilinea, è probabilmente dovuto al senso di visuale, ossia sarebbero viste di taglio. Le stelle e il gas che comprendono le code sarebbero state spinte dal disco galattico (o altre estremità) facilmente distorte di una delle due galassie, piuttosto che dai centri galattici, maggiormente legati dalla gravità. <ref name="toomre72"></ref> Due esempi notevoli di collisioni galattiche che hanno formato code mareali sono le Galassie Topo e le Galassie Antenne.

<!--
thumb|300 px|The [[Mice Galaxies NGC 4676]]
Just as the Moon raises two water tides on opposite sides of the Earth, so a galactic tide produces two arms in its galactic companion. While a large tail is formed if the perturbed galaxy is equal to or less massive than its partner, if it is significantly more massive than the perturbing galaxy, then the trailing arm will be relatively minor, and the leading arm, sometimes called a '''bridge''', will be more prominent.<ref name="toomre72"/> Tidal bridges are typically harder to distinguish than tidal tails: in the first instance, the bridge may be absorbed by the passing galaxy or the resulting merged galaxy, making it visible for a shorter duration than a typical large tail. Secondly, if one of the two galaxies is in the foreground, then the second galaxy — and the bridge between them — may be partially obscured. Together, these effects can make it hard to see where one galaxy ends and the next begins. '''Tidal loops''', where a tail joins with its parent galaxy at both ends, are rarer still.<ref></ref>

=== Satellite interactions ===
thumb|left|300 px|The [[Andromeda Galaxy. Note its satellite galaxy M32 (top left), whose outer arms have been stripped away by Andromeda's tidal forces.]]
Because tidal effects are strongest in the immediate vicinity of a galaxy, satellite galaxies are particularly likely to be affected. Such an external force upon a satellite can produce ordered motions within it, leading to large-scale observable effects: the interior structure and motions of a dwarf satellite galaxy may be severely affected by a galactic tide, inducing rotation (as with the tides of the Earth's oceans) or an anomalous mass-to-luminosity ratio. <ref></ref> Satellite galaxies can also be subjected to the same tidal stripping that occurs in galactic collisions, where stars and gas are torn from the extremities of a galaxy, possibly to be absorbed by its companion. The dwarf galaxy Andromeda, may have lost its spiral arms due to tidal stripping, while a high star formation rate in the remaining core may be the result of tidally-induced motions of the remaining molecular clouds<ref name="Bekkietal2001">{{cite journal
  | author=Bekki, Kenji; Couch, Warrick J.; Drinkwater, Michael J.; Gregg, Michael D.
  | title=A New Formation Model for M32: A Threshed Early-Type Spiral Galaxy?
  | journal=The Astrophysical Journal
  | year=2001
  | volume=557
  | pages=Issue 1, pp. L39–L42
  | url=http://adsabs.harvard.edu/abs/2001ApJ...557L..39B
  | doi=10.1086/323075 }}</ref> (Because tidal forces can knead and compress the interstellar gas clouds inside galaxies, they induce large amounts of star formation in small satellites. The process is somewhat similar to making something hotter by squeezing it).

The stripping mechanism is the same as between two comparable galaxies, although its comparatively weak gravitational field ensures that only the satellite, not the host galaxy, is affected. If the satellite is very small compared to the host, the tidal debris tails produced are likely to be symmetric, and follow a very similar orbit, effectively tracing the satellite's path.<ref></ref> However, if the satellite is reasonably large - typically over one ten thousandth the mass of its host - then the satellite's own gravity may affect the tails, breaking the symmetry and accelerating the tails in different directions. The resulting structure is dependent on both the mass and orbit of the satellite, and the mass and structure of the conjectured galactic halo around the host, and may provide a means of probing the potential of a galaxy such as the Milky Way.<ref></ref>