# 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. 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.[1][2]

Some ultra diffuse galaxies found in the Coma Cluster, about 330 million light years from Earth, have diameters of 60 kly (18 kpc) (more than half the size of our galaxy) with 1% of the stars of the Milky Way Galaxy.[3] 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.[1]

Dragonfly 44,[2] a 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.[4] On 25 August 2016, astronomers reported that Dragonfly 44 may be made almost entirely of dark matter.[5][6][7] In 2018 the same authors reported[8] the discovery of a dark matter-free UDG (NGC 1052-DF2, which was already identified on photoplates by Igor Karachentsev[9]) 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.

• Michael A. Beasley; Aaron J. Romanowsky; Vincenzo Pota; Ignacio Martin Navarro; David Martinez Delgado; Fabian Neyer; Aaron Deich (1 February 2016). "An Overmassive Dark Halo around an Ultra-diffuse Galaxy in the Virgo Cluster". The Astrophysical Journal Letters (published 1 March 2016). 819 (2): L20. arXiv:1602.04002. Bibcode:2016arXiv160204002V. doi:10.3847/2041-8205/819/2/L20.
• Pieter G. van Dokkum; Aaron J. Romanowsky; Roberto Abraham; Jean P. Brodie; Charlie Conroy; Marla Geha; Allison Merritt; Alexa Villaume; Jielai Zhang (6 February 2015). "Spectroscopic Confirmation of the Existence of Large, Diffuse Galaxies in the Coma Cluster". The Astrophysical Journal Letters (published 1 May 2015). 804 (1): L26. arXiv:1504.03320. Bibcode:2015ApJ...804L..26V. doi:10.1088/2041-8205/804/1/L26.
• Koda, Jin; Yagi, Masafumi; Yamanoi, Hitomi; Komiyama, Yutaka (4 June 2015). "Approximately A Thousand Ultra Diffuse Galaxies in the Coma cluster". The Astrophysical Journal Letters. 807 (1): L2. arXiv:1506.01712. Bibcode:2015ApJ...807L...2K. doi:10.1088/2041-8205/807/1/L2.

## References

1. ^ a b
2. ^ a b "Scientists discover the fluffiest galaxies". phys.org. 14 May 2015.
3. ^ "Scientists at Keck Discover the Fluffiest Galaxies". Space Daily. 18 May 2015.
4. ^ Crosswell, Ken (26 July 2016). "The Milky Way's dark twin revealed". Nature News. Retrieved 30 July 2016.
5. ^ Van Dokkum, Pieter; et al. (25 August 2016). "A High Stellar Velocity Dispersion and ~100 Globular Clusters For The Ultra-Diffuse Galaxy Dragonfly 44". The Astrophysical Journal Letters. 828 (1): L6. arXiv:1606.06291. Bibcode:2016ApJ...828L...6V. doi:10.3847/2041-8205/828/1/L6.
6. ^ Hall, Shannon (25 August 2016). "Ghost galaxy is 99.99 per cent dark matter with almost no stars". New Scientist. Retrieved 27 August 2016.
7. ^ Feltman, Rachael (26 August 2016). "A new class of galaxy has been discovered, one made almost entirely of dark matter". The Washington Post. Retrieved 26 August 2016.
8. ^ van Dokkum et al. 2018, Nature volume 555, pages 629–632, "A galaxy lacking dark matter"
9. ^ Karachentsev, I. D., Karachentseva, V. E., Suchkov, A. A. & Grebel, E. K. Dwarf galaxy candidates found on the SERC EJ sky survey. Astron. Astrophys. Suppl. Ser. 145, 415–423 (2000).
2016 in science

A number of significant scientific events occurred in 2016. The United Nations declared 2016 the International Year of Pulses.

Antlia 2

Antlia 2 (Ant 2) is a low-surface-brightness dwarf satellite galaxy of the Milky Way at a galactic latitude of 11.2°. It spans 1.26° in the sky just southeast of Epsilon Antliae. The galaxy is similar in size to the Large Magellanic Cloud, despite being 10,000 times fainter. Antlia 2 has the lowest surface brightness of any galaxy discovered and is ~ 100 times more diffuse than any known ultra diffuse galaxy. It was discovered by the European Space Agency's Gaia spacecraft in November 2018.

Cetus

Cetus () is a constellation. Its name refers to Cetus, a sea monster in Greek mythology, although it is often called 'the whale' today. Cetus is located in the region of the sky that contains other water-related constellations such as Aquarius, Pisces, and Eridanus.

Dark galaxy

A dark galaxy is a hypothesized galaxy with no, or very few, stars. They received their name because they have no visible stars, but may be detectable if they contain significant amounts of gas. Astronomers have long theorized the existence of dark galaxies, but there are no confirmed examples to date. Dark galaxies are distinct from intergalactic gas clouds caused by galactic tidal interactions, since these gas clouds do not contain dark matter, so they do not technically qualify as galaxies. Distinguishing between intergalactic gas clouds and galaxies is difficult; most candidate dark galaxies turn out to be tidal gas clouds. The best candidate dark galaxies to date include HI1225+01, AGC229385, and numerous gas clouds detected in studies of quasars.

On 25 August 2016, astronomers reported that Dragonfly 44, an ultra diffuse galaxy (UDG) with the mass of the Milky Way galaxy, but with nearly no discernable stars or galactic structure, may be made almost entirely of dark matter.

Dragonfly 44

Dragonfly 44 is an ultra diffuse galaxy in the Coma Cluster. Observations of the velocity dispersion suggest a mass of about one trillion solar masses, about the same as the mass of the Milky Way; the galaxy shows no evidence of rotation. 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. The galaxy was discovered with the Dragonfly Telephoto Array.To determine the amount of dark matter in this galaxy, they used the DEIMOS instrument installed on Keck II to measure the velocities of stars for 33.5 hours over a period of six nights so they could determine the galaxy’s mass.

The scientists then used the Gemini Multi-Object Spectrograph on the 8-m Gemini North telescope to reveal a halo of spherical clusters of stars around the galaxy’s core.In August 2016, astronomers reported that this galaxy might be made almost entirely of dark matter.

Galaxy

A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. The word galaxy is derived from the Greek galaxias (γαλαξίας), literally "milky", a reference to the Milky Way. Galaxies range in size from dwarfs with just a few hundred million (108) stars to giants with one hundred trillion (1014) stars, each orbiting its galaxy's center of mass.

Galaxies are categorized according to their visual morphology as elliptical, spiral, or irregular. Many galaxies are thought to have supermassive black holes at their centers. The Milky Way's central black hole, known as Sagittarius A*, has a mass four million times greater than the Sun. As of March 2016, GN-z11 is the oldest and most distant observed galaxy with a comoving distance of 32 billion light-years from Earth, and observed as it existed just 400 million years after the Big Bang.

Research released in 2016 revised the number of galaxies in the observable universe from a previous estimate of 200 billion (2×1011) to a suggested 2 trillion (2×1012) or more, containing more stars than all the grains of sand on planet Earth. Most of the galaxies are 1,000 to 100,000 parsecs in diameter (approximately 3000 to 300,000 light years) and separated by distances on the order of millions of parsecs (or megaparsecs). For comparison, the Milky Way has a diameter of at least 30,000 parsecs (100,000 LY) and is separated from the Andromeda Galaxy, its nearest large neighbor, by 780,000 parsecs (2.5 million LY).

The space between galaxies is filled with a tenuous gas (the intergalactic medium) having an average density of less than one atom per cubic meter. The majority of galaxies are gravitationally organized into groups, clusters, and superclusters. The Milky Way is part of the Local Group, which is dominated by it and the Andromeda Galaxy and is part of the Virgo Supercluster. At the largest scale, these associations are generally arranged into sheets and filaments surrounded by immense voids. The largest structure of galaxies yet recognised is a cluster of superclusters that has been named Laniakea, which contains the Virgo supercluster.

Low Surface Brightness galaxy

A low-surface-brightness galaxy, or LSB galaxy, is a diffuse galaxy with a surface brightness that, when viewed from Earth, is at least one magnitude lower than the ambient night sky.

Most LSBs are dwarf galaxies, and most of their baryonic matter is in the form of neutral gaseous hydrogen, rather than stars. They appear to have over 95% of their mass as non-baryonic dark matter. There appears to be no supernova activity in these galaxies.Rotation curve measurements indicate an extremely high mass-to-light ratio, meaning that stars and luminous gas contribute only very little to the overall mass balance of an LSB. The centers of LSBs show no large overdensities in stars, unlike e.g. the bulges of normal spiral galaxies. Therefore, they seem to be dark-matter-dominated even in their centers, which makes them excellent laboratories for the study of dark matter.

In comparison to the high-surface-brightness galaxies, LSBs are mainly isolated field galaxies, found in regions devoid of other galaxies. In their past, they had fewer tidal interactions or mergers with other galaxies, which could have triggered enhanced star formation. This is an explanation for the small stellar content.

LSB galaxies were theorized to exist in 1976 by Mike Disney.

NGC 1052-DF2

NGC 1052-DF2 is an ultra diffuse galaxy in the constellation Cetus, which was identified in a wide-field imaging survey of the NGC 1052 group by the Dragonfly Telephoto Array. It has been proposed that the galaxy contains little or no dark matter, the first such discovery. However, follow-up studies have not confirmed this.

The distance to the galaxy from Earth, derived by surface brightness fluctuation, is 19 ${\displaystyle \pm }$ 1.7 Mpc. Due to close proximity, it is assumed to be associated with the elliptical galaxy NGC 1052 and to lie at a distance of about 80 Kpc from NGC 1052..

The apparent lack of dark matter in NGC 1052-DF2 may help prove that dark matter is real: if what appears to be dark matter is really just a currently unknown effect of the gravity of ordinary matter then this apparent dark matter should also appear in this galaxy. Further study will be needed before this and any other possible implications can be confirmed. If confirmed, the absence of dark matter may also have implications for theories of galaxy formation, as dark matter has been thought to be needed for galaxy formation.

A later study has shown the galaxy may contain more dark matter than initially reported. It may have a mass-to-light ratio towards the low end of expected values for a dwarf galaxy.

Morphology
Structure
Active nuclei
Energetic galaxies
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