Ultra diffuse galaxy

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NGC 1052-DF2, an ultra diffuse galaxy. NGC 1052-DF2 a ghostly galaxy lacking dark matter.jpg
NGC 1052-DF2, an ultra diffuse galaxy.

An ultra diffuse galaxy (UDG), or Dark galaxy , [1] 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. [lower-alpha 1] 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. [3] [4]

Contents

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 (such a galaxy may have the same size and mass as the Milky Way but a visible star count of only 1%), [5] while other UDGs appear to be almost entirely free of dark matter. [6]

Examples

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. [7] 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. [3]

Dragonfly 44, an ultra diffuse galaxy in the Coma Cluster, is one example. [4] Observations of its 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. [8] On 25 August 2016, astronomers reported that Dragonfly 44 may be made almost entirely of dark matter. [9] [5] [10] However, later, spatially resolved kinematics measured a mass of about 160 billion solar mass, six times less than early mass measurements and 1 order of magnitude less than the Milky Way mass. [11] The most recent work found 20 globular clusters around the galaxy, which is consistent with the recent mass measurement. [12] [13] The lack of X-ray emissions from the galaxy and surrounding area also show that the number of globular clusters can not be as many as was claimed before. [14]

In 2018, the same authors reported the discovery that the ultra diffuse galaxy NGC 1052-DF2 [lower-alpha 2] is dark matter-free, based on velocity measurements of its ~10  globular cluster system. [16] [6] They concluded that this may rule out some alternative gravity theories like modified Newtonian dynamics , but leaves other theories, such as the external field effect still possible. Detailed simulations in the framework of Modified Newtonian Dynamics confirm that NGC 1052-DF2 is quite consistent with theoretical expectations. [17]

In 2021, AGC 114905, an ultra-diffuse dwarf galaxy about 250 million light-years away, was reported to have almost no dark matter. [18] However, this conclusion relies heavily on the galaxy having a moderate inclination of 32° between disc and sky planes, which is estimated from the somewhat oval appearance. Using detailed simulations of AGC 114905 in the alternative gravity theory known as Modified Newtonian Dynamics, it was shown that a disc galaxy with its properties can appear slightly oval even if viewed face-on due to disc self-gravity, in which case the rotation curve could be much higher and the galaxy could be quite consistent with theoretical expectations. [19] An overestimated inclination is unlikely if galaxies are dominated by dark matter because then the disc is not self-gravitating, so it should be close to circular when viewed face-on. [20]

See also

Footnotes

  1. ↑ UDGs are a subset of dwarf spheroidals and dwarf ellipticals and as such are a redundant name for an already well studied galaxy type. [2]
  2. ↑ NGC 1052-DF2 had previously been identified on photographic plates by Karachentsev et al. in 2000. [15]

Related Research Articles

<span class="mw-page-title-main">Galaxy</span> Large gravitationally bound system of stars and interstellar matter

A galaxy is a system of stars, stellar remnants, interstellar gas, dust, and dark matter bound together by gravity. The word is derived from the Greek galaxias (γαλαξίας), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. Galaxies, averaging an estimated 100 million stars, range in size from dwarfs with less than a thousand stars, to the largest galaxies known – supergiants with one hundred trillion stars, each orbiting its galaxy's center of mass. Most of the mass in a typical galaxy is in the form of dark matter, with only a few percent of that mass visible in the form of stars and nebulae. Supermassive black holes are a common feature at the centres of galaxies.

<span class="mw-page-title-main">Local Group</span> Group of galaxies that includes the Milky Way

The Local Group is the galaxy group that includes the Milky Way, where Earth is located. It has a total diameter of roughly 3 megaparsecs (10 million light-years; 9×1019 kilometres), and a total mass of the order of 2×1012 solar masses (4×1042 kg). It consists of two collections of galaxies in a "dumbbell" shape; the Milky Way and its satellites form one lobe, and the Andromeda Galaxy and its satellites constitute the other. The two collections are separated by about 800 kiloparsecs (3×10^6 ly; 2×1019 km) and are moving toward one another with a velocity of 123 km/s. The group itself is a part of the larger Virgo Supercluster, which may be a part of the Laniakea Supercluster. The exact number of galaxies in the Local Group is unknown as some are occluded by the Milky Way; however, at least 80 members are known, most of which are dwarf galaxies.

<span class="mw-page-title-main">Messier 87</span> Elliptical galaxy in the Virgo Galaxy Cluster

Messier 87 is a supergiant elliptical galaxy in the constellation Virgo that contains several trillion stars. One of the largest and most massive galaxies in the local universe, it has a large population of globular clusters—about 15,000 compared with the 150–200 orbiting the Milky Way—and a jet of energetic plasma that originates at the core and extends at least 1,500 parsecs, traveling at a relativistic speed. It is one of the brightest radio sources in the sky and a popular target for both amateur and professional astronomers.

<span class="mw-page-title-main">Dwarf galaxy</span> Small galaxy composed of up to several billion stars

A dwarf galaxy is a small galaxy composed of about 1000 up to several billion stars, as compared to the Milky Way's 200–400 billion stars. The Large Magellanic Cloud, which closely orbits the Milky Way and contains over 30 billion stars, is sometimes classified as a dwarf galaxy; others consider it a full-fledged galaxy. Dwarf galaxies' formation and activity are thought to be heavily influenced by interactions with larger galaxies. Astronomers identify numerous types of dwarf galaxies, based on their shape and composition.

A dark galaxy is a hypothesized galaxy with no 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.

<span class="mw-page-title-main">Satellite galaxy</span> Galaxy that orbits a larger galaxy due to gravitational attraction

A satellite galaxy is a smaller companion galaxy that travels on bound orbits within the gravitational potential of a more massive and luminous host galaxy. Satellite galaxies and their constituents are bound to their host galaxy, in the same way that planets within our own solar system are gravitationally bound to the Sun. While most satellite galaxies are dwarf galaxies, satellite galaxies of large galaxy clusters can be much more massive. The Milky Way is orbited by about fifty satellite galaxies, the largest of which is the Large Magellanic Cloud.

The Milky Way has several smaller galaxies gravitationally bound to it, as part of the Milky Way subgroup, which is part of the local galaxy cluster, the Local Group.

<span class="mw-page-title-main">NGC 4449</span> Galaxy in the constellation Canes Venatici

NGC 4449, also known as Caldwell 21, is an irregular Magellanic type galaxy in the constellation Canes Venatici, being located about 13 million light-years away. It is part of the M94 Group or Canes Venatici I Group that is relatively close to the Local Group hosting our Milky Way galaxy.

<span class="mw-page-title-main">NGC 2808</span> Globular cluster in the constellation Carina

NGC 2808 is a globular cluster in the constellation Carina. The cluster belongs to the Milky Way, and is one of our home galaxy's most massive clusters, containing more than a million stars. It is estimated to be 12.5-billion years old.

<span class="mw-page-title-main">NGC 5286</span> Globular cluster in the constellation Centaurus

NGC 5286 is a globular cluster of stars located some 35,900 light years away in the constellation Centaurus. At this distance, the light from the cluster has undergone reddening from interstellar gas and dust equal to E(B – V) = 0.24 magnitude in the UBV photometric system. The cluster lies 4 arc-minutes north of the naked-eye star M Centauri. It was discovered by Scottish astronomer James Dunlop, active in Australia, and listed in his 1827 catalog.

<span class="mw-page-title-main">NGC 1277</span> Galaxy in the constellation Perseus

NGC 1277 is a lenticular galaxy in the constellation of Perseus. It is a member of the Perseus Cluster of galaxies and is located approximately 73 Mpc (megaparsecs) or 220 million light-years from the Milky Way. It has an apparent magnitude of about 14.7. It was discovered on December 4, 1875 by Lawrence Parsons, 4th Earl of Rosse.

<span class="mw-page-title-main">NGC 1052</span> Elliptical galaxy in the constellation Cetus

NGC 1052 is an elliptical galaxy in the constellation Cetus. It was discovered on January 10, 1785 by the astronomer William Herschel. It is a member of the eponymous NGC 1052 Group.

Triangulum II is a dwarf galaxy close to the Milky Way Galaxy. Like other dwarf spheroidal galaxies, its stellar population is very old: the galaxy was quenched before 11.5 billion years ago. It contains only 1000 stars, yet is quite massive, having a solar mass to light ratio of 3600. This is an unusually high mass for such a small galaxy.

<span class="mw-page-title-main">Dragonfly 44</span> Galaxy in constellation Coma Berenices

Dragonfly 44 is an ultra diffuse galaxy in the Coma Cluster. This galaxy is well-known because observations of the velocity dispersion in 2016 suggested a mass of about one trillion solar masses, about the same as the Milky Way. This mass was consistent with a count of about 90 and 70 globular clusters observed around Dragonfly 44 in two different studies.

The Dragonfly Telephoto Array is a ground-based optical telescope array developed at the Dunlap Institute for Astronomy & Astrophysics of the University of Toronto in Canada. The array uses a combination of telephoto lenses to observe extragalactic objects. Its main purpose is to take images of ultra-low surface brightness galaxies at visible wavelengths of light. It is well suited for this purpose because its lenses have specially-coated optical glass that reduces scattered light.

<span class="mw-page-title-main">NGC 1052-DF2</span> Ultra diffuse galaxy in the constellation Cetus

NGC 1052-DF2 is an ultra diffuse galaxy (UDG) 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. On 20 March 2019, a follow-up study announcing the discovery of a second UDG lacking dark matter, NGC 1052-DF4, was published.

<span class="mw-page-title-main">NGC 3311</span> Galaxy in the constellation Hydra

NGC 3311 is a super-giant elliptical galaxy located about 190 million light-years away in the constellation Hydra. The galaxy was discovered by astronomer John Herschel on March 30, 1835. NGC 3311 is the brightest member of the Hydra Cluster and forms a pair with NGC 3309 which along with NGC 3311, dominate the central region of the Hydra Cluster.

<span class="mw-page-title-main">Gaia Sausage</span> Remains galaxy merger in the Milky Way

The Gaia Sausage or Gaia Enceladus is the remains of a dwarf galaxy that merged with the Milky Way about 8–11 billion years ago. At least eight globular clusters were added to the Milky Way along with 50 billion solar masses of stars, gas and dark matter. It represents the last major merger of the Milky Way.

<span class="mw-page-title-main">NGC 7492</span> Globular cluster in the constellation Aquarius

NGC 7492 is a globular cluster in the constellation Aquarius. It was discovered by the astronomer William Herschel on September 20, 1786. It resides in the outskirts of the Milky Way, about 80,000 light-years away, more than twice the distance between the Sun and the center of the galaxy, and is a benchmark member of the outer galactic halo. The cluster is immersed in, but does not kinematically belong to, the Sagittarius Stream.

<span class="mw-page-title-main">DGSAT I</span> Ultra diffuse galaxy in the Perseus–Pisces Supercluster

DGSAT I is a quenched, ultra diffuse galaxy (UDG) located on the outskirts of the Pisces-Perseus Supercluster, identified in 2016 during a visual inspection of a full color image of the Andromeda II dwarf galaxy. DGSAT I resides in a low-density environment compared to the densities where UDGs are typically found. Its chemical makeup have led astronomers to propose it was formed during the dawn of the universe when galaxies emerged in a different environment than today.

References

  1. ↑ Overbye, Dennis (26 January 2024). "What Do You Call a Galaxy Without Stars? - To dark matter and dark energy, add dark galaxies — collections of stars so sparse and faint that they are all but invisible". The New York Times . Archived from the original on 26 January 2024.
  2. ↑ Conselice, Christopher J. (March 2018). "Ultra-diffuse galaxies are a subset of cluster dwarf elliptical/spheroidal galaxies". Research Notes of the AAS. 2 (1): 43. arXiv: 1803.06927 . Bibcode:2018RNAAS...2...43C. doi: 10.3847/2515-5172/aab7f6 . ISSN   2515-5172. S2CID   55462455.
  3. 1 2 "Astronomers discover 854 ultra-dark galaxies in the famous Coma Cluster". Science Daily. 22 June 2015.
  4. 1 2 "Scientists discover the fluffiest galaxies". phys.org. 14 May 2015.
  5. 1 2 Hall, Shannon (25 August 2016). "Ghost galaxy is 99.99 per cent dark matter with almost no stars". New Scientist . Retrieved 27 August 2016.
  6. 1 2 van Dokkum; et al. (2018). "A galaxy lacking dark matter". Nature. 555 (7698): 629–632. arXiv: 1803.10237 . Bibcode:2018Natur.555..629V. doi:10.1038/nature25767. PMID   29595770. S2CID   4460376.
  7. ↑ "Scientists at Keck discover the fluffiest galaxies". Space Daily. 18 May 2015.
  8. ↑ Crosswell, Ken (26 July 2016). "The Milky Way's dark twin revealed". Nature News. Retrieved 30 July 2016.
  9. ↑ 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 . S2CID   1275440.
  10. ↑ 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.
  11. ↑ Van Dokkum, Pieter; Wasserman, Asher; Danieli, Shany; Abraham, Roberto; Brodie, Jean; Conroy, Charlie; Forbes, Duncan A.; Martin, Christopher; Matuszewski, Matt; Romanowsky, Aaron J.; Villaume, Alexa (2019). "Spatially Resolved Stellar Kinematics of the Ultra-diffuse Galaxy Dragonfly 44. I. Observations, Kinematics, and Cold Dark Matter Halo Fits, Peter van Dokkum et al. 2019". The Astrophysical Journal. 880 (2): 91. arXiv: 1904.04838 . doi: 10.3847/1538-4357/ab2914 . S2CID   119185644.
  12. ↑ Saifollahi, Teymoor; Trujillo, Ignacio; Beasley, Michael A.; Peletier, Reynier F.; Knapen, Johan H. (2020). ""The number of globular clusters around the iconic UDG DF44 is as expected for dwarf galaxies", Saifollahi et al. 2020". Monthly Notices of the Royal Astronomical Society. arXiv: 2006.14630 . doi: 10.1093/mnras/staa3016 .
  13. ↑ "The puzzle of the strange galaxy made of 99.99% dark matter is solved".
  14. ↑ Bogdan, Akos (2020). ""The American Astronomical Society, find out more The Institute of Physics, find out more The Archetypal Ultra-diffuse Galaxy, Dragonfly 44, Is not a Dark Milky Way", Bogdan 2020". The Astrophysical Journal Letters. arXiv: 2009.07846 . doi: 10.3847/2041-8213/abb886 . S2CID   221761669.
  15. ↑ Karachentsev, I.D.; Karachentseva, V.E.; Suchkov, A.A.; Grebel, E.K. (2000). "Dwarf galaxy candidates found on the SERC EJ sky survey". Astron. Astrophys. Suppl. Ser. 145 (3): 415–423. Bibcode:2000A&AS..145..415K. doi: 10.1051/aas:2000249 .
  16. ↑ Parks, Jake (21 October 2019). "Hubble reveals new evidence for controversial galaxies without dark matter". Discover Magazine.
  17. ↑ Haghi, Hosein; Kroupa, Pavel; Banik, Indranil; Wu, Xufen; Zonoozi, Akram H.; Javanmardi, Behnam; Ghari, Amir; Müller, Oliver; Dabringhausen, Jörg; Zhao, Hongsheng (2019-08-14). "A new formulation of the external field effect in MOND and numerical simulations of ultra-diffuse dwarf galaxies – application to NGC 1052-DF2 and NGC 1052-DF4". Monthly Notices of the Royal Astronomical Society. 487 (2): 2441–2454. arXiv: 1906.03268 . Bibcode:2019MNRAS.487.2441H. doi: 10.1093/mnras/stz1465 . ISSN   0035-8711.
  18. ↑ Evidence emerges for dark-matter free galaxies RAS 2021
  19. ↑ New spin on galaxy rotation saves controversial gravity theory RAS 2022
  20. ↑ https://arxiv.org/abs/2202.08678 arXiv 2022

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