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Dwarf galaxies are galaxies with low masses and low luminosities, typically with absolute magnitudes fainter than -18. They appear to contain a significant amount of dark matter, which distinguishes them from globular clusters which contain no dark matter. Dwarfs have rich and complex star-formation and chemical enrichment histories and vary in size, shape, luminosity, and environment. Much of our understanding of these systems comes from studying dwarfs around the Milky Way and in the Local Group, where their close proximity allows for detailed study of the faintest dwarf galaxies. Recent surveys of nearby galaxies suggest these galaxies are very numerous. In some environments, such as galaxy clusters, the total luminosity from all the dwarf galaxies may approach the luminosity of the brightest galaxy in the system. Approximately 90% of the galaxies within a megaparsec of our galaxy are dwarfs.

History
The two most prominent dwarf galaxies in the night sky are the Magellanic Clouds, which are visible with the naked eye and have been observed since prehistoric times. The first recorded telescopic discoveries of dwarf galaxies are from Charles Messier in 1770 of the Andromeda Galaxy's two prominent dwarfs M32 and M110 (NGC 205). At the time it was not understood whether galaxies were small objects in the Milky Way or if they were large, distant objects similar to the Milky Way itself. These were classified as "faint nebulae" until the Cepheid distance scale, discovered by Henrietta Leavitt in 1912, was used by Edwin Hubble to show that these were extragalactic systems. Harlow Shapley is responsible for discovering Sculptor and Fornax in 1938, the first low surface brightness dwarf spheroidal galaxies to be discovered around the Milky Way. Since then, many more dwarf galaxies have been discovered in our own Local Group of galaxies as well as other nearby groups and clusters where these faint objects can be observed. Recently, large surveys such as the Sloan Digital Sky Survey (SDSS) have lead to a rapid acceleration in the discovery rate of dwarfs in our galactic neighborhood.

Structure and Properties


Though most dwarf galaxies are Dwarf galaxies may lack the spiral structure of large spiral galaxies and the luminosity of their elliptical cousins; however, they exhibit a range of appearances and properties unique from more massive galaxies. Currently, in depth studies of the internal kinematics of dwarfs are only possible for galaxies in the Local Group. It takes excellent resolution and long exposure times to resolve individual stars in other galaxies, limiting observations to only nearby systems at the present time.

In the Local Group, all non-rotating dwarf galaxies have central velocity dispersions greater than 7 km s-1. These dispersions are about equal to the dispersions found in globular clusters, despite the fact that dwarf galaxies are as much as 100-1000x larger in size. This extended size of the dwarf galaxies suggests that their velocity dispersions should be much less than globular clusters, closer to 2 km s-1. This discrepancy can only be resolved if there is dark matter in dwarf galaxies, which increases the mass and therefore the velocity dispersion of galaxies. The presence of dark matter is one of the primary characteristics that separate dwarf galaxies from globular clusters.

Low-luminosity dwarfs tend to be metal-poor and are largely composed of nearly primordial material. The large luminosity range for dwarf galaxies enables the study of how parameters, like metallicity, vary with luminosity. It has been known for a while that more luminous dIrr and dSph galaxies are also the most metal-rich.

Classification
Dwarf galaxies can be separated into two main groups; featureless galaxies with smooth surface brightness profiles and little star formation are classified as dwarf spheroidals or dwarf ellipticals, while galaxies that contain visible structure, gas, and ongoing star formation are either dwarf irregulars or dwarf spirals.

Dwarf Spheroidal (dSph) and Dwarf Elliptical (dE)
Dwarf spheroidals and dwarf ellipticals are largely analogous to globular clusters or regular elliptical galaxies. Of these two classes, dwarf ellipticals are typically more massive, and more luminous than dwarf spheroidals. They have luminosities from upwards of $$ 3 \times 10^{7} L_{\odot} $$ and are of similar size as dwarf spheroidals. Their are therefore more easily detected at large distances than dwarf spheroidals. Both M32 and M110 (NGC 205) around Andromeda are dwarf elliptical galaxies.

Dwarf spheroidals generally have luminosities similar to globular clusters of $$\sim10^{6} L_{\odot} $$, but their radii are much larger (~1000 pc) leading to very low surface brightnesses. They are therefore too faint to detect beyond the local group. The Sculptor and Fornax dwarfs and both examples of dwarf spheroidal galaxies.

In the Local Group, most dSph galaxies galaxies do not contain significant amounts of neutral hydrogen (HI), however a few have detectable emission and contain <0.1% of their mass in interstellar HI. These systems contain red giant branch (RGB) stars that are used as kinematic and metallicity probes due to their high luminosities. With the further improvement in telescope and detector quality in recent years, the study of RGB stars has shown that individual dSphs can host stars with a range of metallicities, ages, and kinematic properties. This suggests that dSphs are not simple stellar population, but it remains unclear exactly how these galaxies form and evolve.

Dwarf Irregular (dIrr)


In contrast to the smooth surface brightness profiles of dwarf spheroidals and ellipticals, dwarf irregular galaxies appear more clumpy and distorted, and often contain bright star forming regions that dominate their appearance. The star forming regions are generally superimposed over a more uniform distribution of older populations of stars. The classification cut in luminosity between irregular galaxies and dwarf irregulars is roughly $$L \sim 10^{8}L_{\odot}$$. Our Milky Way's own Large Magellanic Cloud and Small Magellanic Cloud are examples of dIrr galaxies.

Most of what is known about the kinematics and metalicities of these systems comes from observing their large fractions of HI gas and numerous massive stars. These galaxies are also supported by velocity dispersions of similar magnitudes to other dwarfs; however, this velocity dispersion is influenced by on-going star formation processes. Therefore, the HI velocity dispersions will always be ~10km s-1 regardless of mass or rotation velocity. It is then slightly erroneous to directly compare measured velocity dispersions for dIrr and dSph galaxies.

Observations have shown that 7-50% of the mass of dIrr galaxies in our Local Group can be in the form of neutral hydrogen, and all contain detectable ionized hydrogen (HII). Spectra of the HII regions as well as young massive stars reveals most of the metallicity information. This is in contrast to dSph abundances which are measured in older stars. Therefore, comparing metallicities between dIrr and dSph is also difficult because the properties are measured in different populations of stars.

Formation and Evolution
Much of what is known about dwarf galaxies has come from the study of those in the Local Group. These works have led to a greater understanding of how a galaxy's environment plays a role in its formation and evolution. For now, it is unknown if dSph and dIrr are created differently or whether they are different evolutionary stages of the same type of object. Furthermore, the surrounding environment can significantly influence the appearance of dwarfs. For instance, both NGC 205 and the Sagittarius dwarf show evidence of tidal distortions as a result of interactions with their host galaxy. During strong interactions, stars can be shed into leading and trailing orbits, quickly filling a larger volume than the original galaxy. At later times, the dwarf can become elongated by tidal effects, producing a very long stellar stream along its orbit around the host galaxy. It is possible that a large fraction of the Milky Way's extended halo was built from disrupted dSph systems.

Dwarf galaxies are also useful subjects for study because of their low metallicities. Because of this, dwarfs are assumed to have not experienced as much star formation over their history as larger galaxies. Dwarfs are also of great importance to cosmologists because these systems may be the building blocks of larger systems. Current models of hierarchical structure formation predict that smaller systems formed first and merged into larger galaxies. The low metallicities are puzzling because many dwarfs exhibit fairly high star formation rates. Several mechanisms have been suggested to answer this question. . If the galaxy preferentially formed more high-mass stars compared to other galaxies (a change in the initial mass function), it would result in the formation of fewer stars total and reduce the chemical enrichment of the interstellar medium. Another potential mechanism to reduce the metallicity of the gas would be the accretion metal-poor gas from outside the galaxy, diluting the metal content. Additionally, there may also be metal-rich gas outflows caused by supernovae or galactic winds that expel the enriched gas out of the galaxy. Gas outflows could also originate from tidal interactions with a larger galaxy in a close encounter. Any or all of these mechanisms could be responsible, and there remains significant debate.

Formation of Dwarf Ellipticals
Dwarf elliptical galaxies are thought to have a more complicated formation process than dwarf irregulars. The gas content of dwarf irregulars suggests that they formed via collapse of a gas cloud into a disk, which then formed stars. This naturally produces a thin disk of stars. The spheroidal shape of dwarf ellipticals and spheroidals is inconsistent with such a simple formation scenario. Dwarf ellipticals are also preferentially found near other galaxies or galaxy clusters, and rarely found in isolated environments. This suggests dwarf ellipticals must be the result of some form of processing in dense environments.

It is unclear what the progenitors of of dwarf ellipticals are. Some theories have suggested that they are formed from dwarf irregulars, or even normal spiral galaxies. Upon passing near a more massive galaxy these progenitors could have had their gas stripped, thus ending star formation in the dwarf. The interaction could also alter the kinematics of the dwarf galaxy, altering the orbits of its constituent stars from an ordered disk into a more randomized spheroidal (a process known as "galaxy harassment"). An alternative theory is that dwarf ellipticals are the the remnants of larger elliptical galaxies that have had their outer layers of stars stripped away by interactions with another galaxy. There is no consensus yet on which of these possible formation mechanisms is at work.