What constitutes a galaxy? Over the past several years, the discovery of a number of very small galaxies has motivated some astronomers to reconsider the definition of a galaxy since there is currently no widely accepted standard definition for a galaxy. The tiniest galaxies have masses and sizes that are known to overlap with the largest globular clusters. Basically, a globular cluster is a gravitationally bound spherical collection of stars which can contain anywhere from a few thousand to several million stars. In comparison, a large galaxy such as the Milky Way galaxy contains hundreds of billions of stars, measures approximately 100000 light years in diameter and has a total mass that is around one trillion times the mass of our Sun. There are about 150 or so globular clusters that are currently known to orbit the Milky Way galaxy.
In recent years, the discovery of objects called Ultra Compact Dwarfs has shown that these objects have properties which blur the distinction between the smallest galaxies and the largest globular clusters. Ultra Compact Dwarfs generally contain on the order of one million to a hundred million times the mass of our Sun and they typically have sizes of around several tens of light years across. In the Local Group, Omega Centauri is the largest globular cluster in the Milky Way galaxy while Mayall II is the largest globular cluster in the neighbouring Andromeda galaxy. Omega Centauri is estimated to contain a total of 5 million times the mass of our Sun and Mayall II is believed to be twice as massive as Omega Centauri. These are among the largest globular clusters known and they clearly show properties which overlap with Ultra Compact Dwarfs. It has also been suggested that Omega Centauri and Mayall II may even be the remaining cores of tidally disrupted dwarf galaxies. Furthermore, the smallest ultra-faint Dwarf Spheroidal Galaxies also have properties which overlap with the largest globular clusters.
A number of requirements are discussed here that can help in distinguishing a small dwarf galaxy from a large globular cluster. To start off, the minimum requirement for a galaxy is that it has to be gravitationally bound and it must contain stars. This means that streams of stars and material that were ejected from the collision of galaxies and masses of starless ‘dark galaxies’ cannot be classified as galaxies. However, these requirements will still include all globular clusters and Ultra Compact Dwarfs as galaxies. Hence, a number of additional requirements and their implications will be discussed to better distinguish objects that have properties which are intermediate between the largest globular clusters and the smallest galaxies.
A gravitationally bound stellar system that is in a dynamically stable state will have stars whose orbits are determined by the mean gravitational field of the system rather than by encounters between individual stars. If a galaxy is defined as a dynamically stable gravitationally bound stellar system with a two-body relaxation timescale that is longer than the age of the universe, then Ultra Compact Dwarfs can be classified as galaxies but not globular clusters. In gravitationally bound stellar systems with deep gravitational potential wells, gases enriched with heavy elements created by nucleosynthesis processes from one or more episodes of star formation will remain gravitationally bound to the system. These enriched gases can be available for subsequent episodes of star formation, leading to populations of stars with different abundances of heavier elements. If a galaxy is required to have multiple populations of stars, then almost all Ultra Compact Dwarfs and the most massive globular clusters can be classified as galaxies.
The Milky Way galaxy is embedded within a massive halo of non-baryonic dark matter. If the presence of a massive dark matter halo is required for a stellar system to be classified as a galaxy, then globular clusters, Ultra Compact Dwarfs and some Dwarf Spheroidal Galaxies cannot be classified as galaxies. A known characteristic of all large galaxies such as the Milky Way galaxy and the Andromeda galaxy is that they all have systems of globular clusters and smaller satellite galaxies in orbit around them. If a system of globular clusters is required, then Ultra Compact Dwarfs and some dwarf galaxies will be unable to meet this requirement to be classified as galaxies. Finally, size can also be a requirement for defining what constitutes a galaxy. Ultra Compact Dwarfs generally have sizes of up to just over 100 light years across, while the smallest ultra-faint Dwarf Spheroidal Galaxies have sizes that are as small as 300 light years across. If a minimum size of 300 light years is a requirement for a galaxy, then all Ultra Compact Dwarfs and globular clusters cannot be classified as galaxies.