A galactic tide is a tidal force experienced by objects subject to the gravitational field of a galaxy such as the Milky Way. Particular areas of interest concerning galactic tides include galactic collisions, the disruption of dwarf or satellite galaxies, and the Milky Way’s tidal effect on the Oort cloud of the Solar System.
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 rarely (if ever) collide head-on, 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, which are 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. If a tail appears to be 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. 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.
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.
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, 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 (Because tidal forces can knead and compress the interstellar gas clouds inside galaxies, they induce large amounts of star formation in small satellites.)
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.
Tidal effects are also present within a galaxy, where their gradients are likely to be steepest. This can have consequences for the formation of stars and planetary systems. 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 the Solar System, the hypothetical Oort cloud, believed to be the source of long-period comets, lies in this transitional region.
The Oort cloud is believed to be a vast shell surrounding the 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.