An ultra diffuse galaxy 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. These galaxies have been studied for many years prior to their renaming in 2015. Their lack of luminosity is due to the lack of star-forming gas, which results in these galaxies being reservoirs of very old stellar populations. Based on discoveries confirmed in 2018, this class of galaxies includes both extremes of dark matter content: Some UDGs consist almost entirely of dark matter, while other UDGs are almost entirely free of dark matter.
Examples
Some ultra diffuse galaxies found in the Coma Cluster, about 330 million light years from Earth, have diameters of 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 its rotational speedsuggest a mass of about one trillionsolar 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 that the ultra diffuse galaxy NGC 1052-DF2 is dark matter-free, based on velocity measurements of its ~10 globular cluster system. They concluded that this may rule out some alternate gravity theories like modified Newtonian dynamics, but leaves other theories such as the external field effect still possible.
