A satellite galaxy is a galaxy that orbits a larger one due to gravitational attraction. Accordingly, the masses of two galaxies determine their orbitary relation to each other. Galaxies are made of a large number of objects (such as stars, planets, and nebulae) that are not connected to each other. However, they have a center of mass which represents a weighted average of the positions of each component object. This is similar to how also humans have a center of mass which is the weighted average of the body's component atoms and their masses in relation to their position. Even within a galaxy, the stars orbit the galactic center, also called bulge.
Galaxies which encounter one another from certain directions may interact: collide, merge, rip each other apart, or transfer some member objects. In these situations, it can be difficult to tell where one galaxy ends and where another begins. "Collisions" between galaxies do not necessarily involve collisions between objects from one galaxy and objects from the other, since galaxies are mostly composed of empty space. Rather, gravitational forces from nearby clouds of matter are capable of distorting one another, or bringing a clump of mass from one galaxy over into another.
Relation to primary galaxy
In a pair of orbiting galaxies, the considerably larger one is the "primary" and the smaller is the satellite. The complexity of a satellite galaxy is far greater than that of the one it orbits, as not only its own evolution determines its properties but also the gravitational pull of the primary and the environment it provides.
Many satellite galaxies cannot resist the gravitation and "fall" into the large galaxy. This can be determined by looking at the stellar halo of a satellite that no longer exists. This halo consists of stars that were part of the satellite and got separated from it.
If two orbiting galaxies are about the same size, then they are said to form a binary system.
Nested system of substructure
Satellite galaxies being part of a primary galaxy's system, they may have interactions with other dwarfs. Apart from these subsystems, satellites are able to bring along companions as they infall onto the large galaxy.
The largest satellite galaxy of the Milky Way is the Large Magellanic Cloud (LMC). It is believed that it brought other dwarfs with it, the Small Magellanic Cloud (SMC) being a strong candidate for that.
Enveloping a galactic disk, a dark matter halo has an impact on the galaxy's and its stars' motion. According to current models, dark matter halos are crucial to the formation and evolution of a galaxy.
Satellites can be treated as massless test-particles in order to trace dark matter halos, according to their position and motion throughout time.
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