Ultra diffuse galaxy
An ultra diffuse galaxy (UDG) is an extremely low luminosity galaxy, the first example of which was discovered in the nearby Virgo Cluster by Allan Sandage and Bruno Binggeli in 1984. UDGs are a subset of dwarf spheroidals and dwarf ellipticals and as such are a redundant name for an already well studied galaxy type. Such a galaxy may have the same size and mass as the Milky Way but a visible star count of only 1%. Their lack of luminosity is due to the lack of star-forming gas in the galaxy. This results in old stellar populations.
Some ultra diffuse galaxies found in the Coma Cluster, about 330 million light years from Earth, have diameters of 60 kly (18 kpc) with 1% of the stars of the Milky Way Galaxy. The distribution of ultra diffuse galaxies in the Coma Cluster is the same as luminous galaxies; this suggests that the cluster environment strips the gas from the galaxies, while allowing them to populate the cluster the same as more luminous galaxies. The similar distribution in the higher tidal force zones suggests a larger dark matter fraction to hold the galaxies together under the higher stress.
Dragonfly 44, an ultra diffuse galaxy in the Coma Cluster, is one example. Observations of the rotational speed suggest a mass of about one trillion solar masses, about the same as the mass of the Milky Way. This is also consistent with about 90 globular clusters observed around Dragonfly 44. However, the galaxy emits only 1% of the light emitted by the Milky Way. On 25 August 2016, astronomers reported that Dragonfly 44 may be made almost entirely of dark matter. In 2018 the same authors reported the discovery of a dark matter-free UDG (NGC 1052-DF2, which was already identified on photoplates by Igor Karachentsev) based on velocity measurements of ~10 globular cluster system. The authors concluded that this may rule out modified gravity theories like MOND, but other theories such as the External Field Effect are also possibilities.
- Dark galaxy – A hypothesized galaxy with no, or very few, stars
- Low-surface-brightness galaxy, also known as LSB galaxy
- Type-cD galaxy or c-Diffuse galaxy type
- Type-D galaxy or Diffuse-type galaxy
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