Galactic Tide - Origin

Origin

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 any object A is to another object B, the more intensely A is affected by the object B's gravity, according to Newton's law of universal gravitation. This is also true of the different parts of an object; The surface of object A feels a stronger attraction to object B than the core of object A. 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 ovoid 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.