A galactic tide is a cosmic process in which an object of small mass is distorted gravitationally by one of larger mass, such as a satellite galaxy being influenced by a larger galaxy. This process occurs as gravitational attraction between two objects increases with a decrease in distance, with objects in close proximity to each other experiencing stronger attraction and greater probability of generating a galactic tide.
A good theoretical example of a galactic tide is two galaxies located close to each other, one with a large mass and the other with a smaller mass. Here, the stronger gravitational attraction of the large galaxy causes the outer layers of the smaller one to be siphoned and drawn off from its core, warping and elongating it from a rough disc shape to an uneven one. This shearing tidal force is experienced by the larger galaxy too, however, due to the smaller one’s lesser mass and gravitational attraction – as described in Isaac Newton’s law of universal gravitation – it does not experience the effect as strongly.
Interestingly, galactic tides do not necessarily result in galactic collisions, as tidal forces are dependent on the gradient of gravitational field as well as strength. For example, in the aforementioned example of two galaxies, the result of close proximity will not necessarily lead to a collision as the tidal forces involved will distort each galaxy along an axis both pointing towards and away from its opposite number. As the two galaxies orbit each other, which can be temporarily, these distorted stretched-out regions are sheared off the corresponding cores due to their differential rotation (different layers are rotating at different angular velocities) and ejected into intergalactic space. This ejection generates galactic tails, strongly curved expanses of stars and gas that extend out from the galaxy cores (a good example of this can be seen in the image of the Mice Galaxies on the opposite page).
Galactic tides not only affect the shape and composition of stellar objects but also other factors, too. In small-scale objects such as dwarf galaxies, tidal forces can alter their interior structure and motions, luminosity factor and – in galaxies -star formation rate. A good example of these effects can be seen in the case of dwarf galaxy M32, a satellite of the mighty Andromeda Galaxy. Here, not only has M32 had its spiral arms accreted by Andromeda in a tidal stripping process but also it has an anomalous mass-to-luminosity ratio as well as an abnormally high star formation rate in its core – the latter theorized to be generated by tidally induced motions in its remaining molecular clouds.
While galactic tides occur outside of a galaxy, tidal forces in general occur both outside and inside them, with the gravitationally weak boundaries of systems most prone (the region where the Sun’s gravitational influence is overpowered; referred to as the tidal truncation radius). A good example of this can be visualized with our own Solar System, with the hypothetical Oort cloud that surrounds it being gravitationally distorted by the Milky Way’s tidal forces. This distortion is postulated to be one of the primary generators of comets within our Solar System, with the galactic tide bending and distorting the orbits of bodies (such as comets) and drawing them towards the galactic centre.
Tides in space
Galactic tides are generated by galaxies due to their huge gravitational attraction and tidal forces. As such, satellites of major galaxies are the most common places to find evidence of the galactic tidal processes.
One of the best currently known interactions of this type can be found in the relationship between the dwarf elliptical galaxy M32 and Andromeda. Here, not only has the significantly smaller M32 had almost all of its outer layers (spiral arms) sheared away by the tidal forces of Andromeda but it is also having its core manipulated by them. This manipulation consists of tidally induced motions within its central molecular clouds, with the tidal forces kneading and compressing them on a large scale. This is inducing the creation of large quantities of new stars within the heart of the system and rapidly accelerating their full formation.