Marea galattica: differenze tra le versioni

 

==Origine==

 

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.

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">{{cite journal|title=Galactic Bridges and Tails|author=Toomre A. & Toomre J.|journal=The Astrophysical Journal|volume=178|pages=623–666|year=1972|url=http://adsabs.harvard.edu/abs/1972ApJ...178..623T|doi=10.1086/151823 }}</ref> Two very prominent examples of collisions producing tidal tails are the [[Mice Galaxies]] and the [[Antennae Galaxies]].

 

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>{{cite journal|title=NGC 3310 and its tidal debris: remnants of galaxy evolution|author=Wehner E.H. et al.|journal=Monthly Notices of the Royal Astronomical Society|volume=371|issue=3|pages=1047–1056|year=2006|url=http://adsabs.harvard.edu/abs/2006MNRAS.371.1047W|doi=10.1111/j.1365-2966.2006.10757.x }}</ref>

 

=== Satellite interactions ===

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#In_astronomy|luminosity]] ratio. <ref>{{cite journal|title=Can Galactic Tides Inflate the Apparent M/L's of Dwarf Galaxies?|author=Piatek S. & Pryor C.|journal=Bulletin of the American Astronomical Society|volume=25|pages=1383|year=1993|url=http://adsabs.harvard.edu/abs/1993AAS...183.5701P}}</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 [[M32]], a satellite galaxy of [[Andromeda galaxy|Andromeda]], may have lost its [[spiral arm]]s 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 cloud]]s<ref name="Bekkietal2001">{{cite journal

| author=Bekki, Kenji; Couch, Warrick J.; Drinkwater, Michael J.; Gregg, Michael D.

Tidal effects are also present within a galaxy, where their gradients are likely to be steepest. This can have consequences for the formation of [[star]]s and [[planetary system]]s. Typically a star's gravity will dominate within its own system, with only the passage of other stars substantially affecting dynamics. However, at the outer reaches of the system, the star's gravity is weak and galactic tides may be significant. In our own solar system, the hypothetical [[Oort cloud]], believed to be the source of long-period [[comet]]s, lies in this transitional region.

 

 

The Oort cloud is believed to be a vast shell surrounding our solar system, possibly over a [[light-year]] in radius. Across such a vast distance, the gradient of the Milky Way's gravitational field plays a far more noticeable role. Because of this gradient, galactic tides may then deform an otherwise spherical Oort cloud, stretching the cloud in the direction of the galactic centre and compressing it along the other two axes, just as the Earth distends in response to the gravity of the Moon.