Scaling relations describe strong trends that are observed between important physical properties (such as mass, size, luminosity and colours) of galaxies. Both “early-type“cosmos/E/early-type+galaxies (elliptical and S0) and late-type (spiral) galaxies exhibit scaling relations, though different relations are used for each type.
For example, there are a number of important scaling relations for early-type galaxies. These include:
There are also scaling relations for late-type galaxies, the most important of which is the Tully-Fisher Relation. This relation, which can be derived from the virial theorem, relates the rotation speed of the galaxy to its luminosity, and is often used to determine distances in the Universe. It therefore provides an important rung in the distance ladder.
The existence of scaling relations that correlate the physical properties of widely separated galaxies, indicates that the formation processes for all galaxies within a particular galaxy type must be fairly similar. The relations therefore provide insights into both the formation and evolution of galaxies, and many are also used to measure the distances to galaxies.