The Andromeda Galaxy. Note its satellite galaxy M32 (top left), whose outer arms have been stripped away by Andromeda's tidal forces

A galactic tide is a tidal force subjected on objects by the gravitational field of a galaxy such as the Milky Way. Much like tides on Earth, which are primarily caused by regional differentiation in the gravitational force of the Moon, so a body in close proximity to (or embedded in) a galaxy will feel tidal forces from its immense gravity. This can cause it to distend towards the centre of the galaxy, or suffer perturbations to its orbit. 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 our own solar system. Image File history File links M31_Lanoue. ... Image File history File links M31_Lanoue. ... M31 in a small telescope The Andromeda Galaxy (IPA: , also known as Messier 31, M31, or NGC 224; older texts often called it the Andromeda Nebula) is a spiral galaxy approximately 2. ... Elliptical Galaxy M32 (also known as Messier Object 32, Messier 32, M32, or NGC 221) is a dwarf elliptical galaxy in the Andromeda constellation, a satellite of the Andromeda Galaxy, and a member of the Local Group galaxies. ... Comet Shoemaker-Levy 9 after breaking up under the influence of Jupiters tidal forces. ... The gravitational field is a field (physics), generated by massive objects, that determines the magnitude and direction of gravitation experienced by other massive objects. ... NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, is about 17,000 parsecs in diameter and approximately 20 million parsecs distant. ... The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias), sometimes referred to simply as the Galaxy), is a barred spiral galaxy of the Local Group. ... It has been suggested that Earth tides be merged into this article or section. ... Interacting galaxies (Colliding galaxies) is the result of gravity of two close galaxies. ... A dwarf galaxy is a small galaxy composed of up to several billion stars, a small number compared to our own Milky Ways 200-400 billion stars. ... A satellite galaxy orbits a larger galaxy, due to gravitational attraction. ... This image is an artists rendering of the Oort cloud and the Kuiper Belt. ... Major features of the Solar System (not to scale, from left to right): Pluto, Neptune, Uranus, Saturn, a comet, Jupiter, the asteroid belt, the Sun, Mercury, Venus, Earth & Moon, and Mars. ...

Effects on external galaxies

Because tidal forces are dependent on the gradient of a gravitational field, rather than its strength, tidal effects are usually limited to the immediate surroundings of a galaxy; this is why 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 kinematics 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.^[1] The dwarf galaxy M32, a satellite galaxy of 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.^[2] In physics, kinematics is the branch of classical mechanics concerned with describing the motions of objects without considering the factors that cause or affect the motion. ... Luminosity has different meanings in several different fields of science. ... M32 refers to: Messier 32 a Messier object and an elliptical galaxy in the Andromeda constellation. ... M31 in a small telescope The Andromeda Galaxy (IPA: , also known as Messier 31, M31, or NGC 224; older texts often called it the Andromeda Nebula) is a spiral galaxy approximately 2. ... A spiral galaxy presents a face-on view of its spiral arms. ... A molecular cloud is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen (H2). ...

Over many orbits of its parent galaxy, or if the orbit passes too close to it, a dwarf satellite may even be completely disrupted, to form a tidal stream of stars and gas tracing its previous path. It has been suggested that the extended discs of gas and stars around some galaxies, such as Andromeda, may be the result of the complete tidal disruption (and subsequent merger with the parent galaxy) of a dwarf satellite galaxy.^[3]

The Mice Galaxies NGC 4676

Two large galaxies undergoing collisions or passing nearby each other will also be subjected to very large tidal forces, resulting in a 'tail' of stars that are stripped away from the galactic discs (or other extremeties) of one or both bodies. 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.^[4] Such tails are typically strongly curved; where a tail appears straight, it is probably being viewed edge-on. Two very prominent examples of collisions producing tidal tails are the Mice Galaxies and the Antennae Galaxies. Image File history File linksMetadata Download high resolution version (2843x1312, 402 KB) Summary NGC 4676, The Mice galaxies. ... Image File history File linksMetadata Download high resolution version (2843x1312, 402 KB) Summary NGC 4676, The Mice galaxies. ... Differential rotation is seen if parts of a rotating object move with different angular velocity. ... Intergalactic space is the physical space between galaxies. ... The Mice Galaxies ar two spiral galaxies in the constellation Coma Berenices. ... The Antennae Galaxies (also known as NGC 4038/NGC 4039) are a pair of galaxies about 68 million ly away in the constellation Corvus. ...

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 then the trailing arm will be relatively minor, and the leading arm, or bridge, will be more prominent.^[4] 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.^[5]

Effects on bodies within a galaxy

Tidal effects are also significant within a galaxy, where its gradient is likely to be steepest. This can have consequences for the formation of stars and planetary systems. Typically a star's gravity will dominate within such a system, with only other stars passing close by affecting the dynamics of the system. 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 comets, lies in this transitional region. This article is about the astronomical object. ... An artists concept of a protoplanetary disc. ... Comet Hale-Bopp Comet McNaught as seen from Swifts Creek, Victoria, Australia on 23 January 2007 A comet is a small body in the solar system that orbits the Sun and (at least occasionally) exhibits a coma (or atmosphere) and/or a tail â€” both primarily from the effects of...

Diagram of the Oort cloud.

The Oort cloud is hypothesized to be a vast (possibly over a light-year in radius) shell surrounding our solar system, and the variation of the Milky Way's gravitational field over this distance is not insignificant. 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. The mechanism is the same as that by which the Moon produces the tides on Earth. Image File history File links Kuiper_oort. ... Image File history File links Kuiper_oort. ... This image is an artists rendering of the Oort cloud and the Kuiper Belt. ... A light-year, symbol ly, is the distance light travels in one year: exactly 9. ...

The Sun's gravity is sufficiently weak at such a distance that these small galactic perturbations may be enough to dislodge some planetesimals from such distant orbits, sending them towards the Sun and planets by significantly reducing their perihelion.^[6] Such a body, being composed of a rock and ice mixture, would become a comet when subjected to the increased solar radiation present in the inner solar system. In cosmogony, planetesimals are objects thought to exist within solar nebulae. ... This article is about several astronomical terms (apogee & perigee, aphelion & perihelion, generic equivalents based on apsis, and related but rarer terms. ...

It has been suggested that the galactic tide may also contribute to the formation of an Oort cloud, by increasing the perihelion of planetesimals with large aphelion.^[7] This shows that the effects of the galactic tide are quite complex, and depend heavily on the behaviour of individual objects within a planetary system. Cumulatively the effect can be quite significant, however; up to 90% of all comets originating from an Oort cloud may be the result of the galactic tide.^[8] This article is about several astronomical terms (apogee & perigee, aphelion & perihelion, generic equivalents based on apsis, and related but rarer terms. ...

See also

• Oort cloud
• Roche limit
• Satellite galaxy
• Dwarf galaxy
• Interacting galaxy
• Tidal force

This image is an artists rendering of the Oort cloud and the Kuiper Belt. ... The Roche limit, sometimes referred to as the Roche radius, is the distance within which a celestial body held together only by its own gravity will disintegrate due to a second celestial bodys tidal forces exceeding the first bodys gravitational self-attraction. ... A satellite galaxy orbits a larger galaxy, due to gravitational attraction. ... A dwarf galaxy is a small galaxy composed of up to several billion stars, a small number compared to our own Milky Ways 200-400 billion stars. ... Interacting galaxies (Colliding galaxies) is the result of gravity of two close galaxies. ... Comet Shoemaker-Levy 9 after breaking up under the influence of Jupiters tidal forces. ...

References

1. ^ Piatek S. & Pryor C. (1993). "Can Galactic Tides Inflate the Apparent M/L's of Dwarf Galaxies?". Bulletin of the American Astronomical Society 25: 1383. 
2. ^ Bekki, Kenji; Couch, Warrick J.; Drinkwater, Michael J.; Gregg, Michael D. (2001). "A New Formation Model for M32: A Threshed Early-Type Spiral Galaxy?". The Astrophysical Journal 557: Issue 1, pp. L39-L42. 
3. ^ Peñarrubia J., McConnachie A. & Babul A. (2006). "On the Formation of Extended Galactic Disks by Tidally Disrupted Dwarf Galaxies". The Astrophysical Journal. 
4. ^ ^{a b} Toomre A. & Toomre J. (1972). "Galactic Bridges and Tails". The Astrophysical Journal 178: 623-666. 
5. ^ Wehner E.H. et al. (2006). "NGC 3310 and its tidal debris: remnants of galaxy evolution". Monthly Notices of the Royal Astronomical Society 371 (3): 1047-1056. 
6. ^ Fouchard M. et al. (2006). "Long-term effects of the Galactic tide on cometary dynamics". Celestial Mechanics and Dynamical Astronomy. 
7. ^ Higuchi A., Kokubo E. & Mukai, T. (2005). "Orbital Evolution of Planetesimals by the Galactic Tide". Bulletin of the American Astronomical Society. 
8. ^ Nurmi P., Valtonen M.J. & Zheng J.Q. (2001). "Periodic variation of Oort Cloud flux and cometary impacts on the Earth and Jupiter". Monthly Notices of the Royal Astronomical Society 327: 1367-1376.