Ultra diffuse galaxy

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

An ultra diffuse galaxy (UDG), or dark galaxy,{{cite news |last=Overbye |first=Dennis |authorlink=Dennis Overbye|title=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. |url=https://www.nytimes.com/2024/01/26/science/space/dark-galaxy-stars.html |date=26 January 2024 |work=The New York Times |url-status=live |archiveurl=https://archive.today/20240126135818/https://www.nytimes.com/2024/01/26/science/space/dark-galaxy-stars.html |archivedate=26 January 2024 }} 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.{{efn|UDGs are a subset of dwarf spheroidals and dwarf ellipticals and as such are a redundant name for an already well studied galaxy type.{{cite journal |last=Conselice |first=Christopher J. |date=March 2018 |title=Ultra-diffuse galaxies are a subset of cluster dwarf elliptical/spheroidal galaxies |journal=Research Notes of the AAS |volume=2 |issue=1 |pages=43 |doi=10.3847/2515-5172/aab7f6 |issn=2515-5172 |arxiv=1803.06927 |bibcode=2018RNAAS...2...43C|s2cid=55462455 |doi-access=free }}}} 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. These galaxies with lower luminosity are more likely to be found in clusters, such as the coma cluster, than in groups.{{Cite journal |last1=Burg |first1=Remco F. J. van der |last2=Hoekstra |first2=Henk |last3=Muzzin |first3=Adam |last4=Sifón |first4=Cristóbal |last5=Viola |first5=Massimo |last6=Bremer |first6=Malcolm N. |last7=Brough |first7=Sarah |last8=Driver |first8=Simon P. |last9=Erben |first9=Thomas |last10=Heymans |first10=Catherine |last11=Hildebrandt |first11=Hendrik |last12=Holwerda |first12=Benne W. |last13=Klaes |first13=Dominik |last14=Kuijken |first14=Konrad |last15=McGee |first15=Sean |date=2017-11-01 |title=The abundance of ultra-diffuse galaxies from groups to clusters - UDGs are relatively more common in more massive haloes |url=https://www.aanda.org/articles/aa/abs/2017/11/aa31335-17/aa31335-17.html |journal=Astronomy & Astrophysics |language=en |volume=607 |pages=A79 |doi=10.1051/0004-6361/201731335 |issn=0004-6361|arxiv=1706.02704 |bibcode=2017A&A...607A..79V }} Ultra diffuse galaxies have a similar size to the Milky Way, but they have masses more akin to dwarf galaxies.{{Citation |last1=Benavides |first1=José A. |title=Quiescent ultra-diffuse galaxies in the field originating from backsplash orbits |date=2021-09-03 |url=https://arxiv.org/abs/2109.01677 |access-date=2025-02-03 |arxiv=2109.01677 |last2=Sales |first2=Laura V. |last3=Abadi |first3=Mario G. |last4=Pillepich |first4=Annalisa |last5=Nelson |first5=Dylan |last6=Marinacci |first6=Federico |last7=Cooper |first7=Michael |last8=Pakmor |first8=Ruediger |last9=Torrey |first9=Paul|journal=Nature Astronomy |volume=5 |issue=12 |pages=1255–1260 |doi=10.1038/s41550-021-01458-1 |bibcode=2021NatAs...5.1255B }} The formation of ultra diffuse galaxies is still debated, however they're thought to have formed from the earliest of galaxies or tidal forces.{{Cite web |title=Creating Ultra-Diffuse Galaxies {{!}} Center for Astrophysics {{!}} Harvard & Smithsonian |url=https://www.cfa.harvard.edu/news/creating-ultra-diffuse-galaxies |access-date=2025-02-03 |website=www.cfa.harvard.edu |language=en}}

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%), while other UDGs appear to be 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 {{convert|60|kly|kpc|abbr=on|lk=on}} with 1% of the stars of the Milky Way Galaxy.{{cite news |url=http://www.spacedaily.com/reports/Scientists_at_Keck_Discover_the_Fluffiest_Galaxies_999.html |title=Scientists at Keck discover the fluffiest galaxies |date=18 May 2015 |publisher=Space Daily }} 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.{{cite news |url=https://www.sciencedaily.com/releases/2015/06/150622162027.htm |title= Astronomers discover 854 ultra-dark galaxies in the famous Coma Cluster |publisher=Science Daily |date=22 June 2015}}

Dragonfly 44, an ultra diffuse galaxy in the Coma Cluster, is one example.{{cite web |url=http://phys.org/news/2015-05-scientists-fluffiest-galaxies.html |title=Scientists discover the fluffiest galaxies |date=14 May 2015 |publisher=phys.org}} It was first discovered by the Coma Cluster survey which specifically scanned for very low surface brightness objects via the multi beam optical Dragonfly Telephoto Array.{{Cite journal |last1=Pozo |first1=Alvaro |last2=Broadhurst |first2=Tom |last3=de Martino |first3=Ivan |last4=Luu |first4=Hoang Nhan |last5=Smoot |first5=George F |last6=Lim |first6=Jeremy |last7=Neyrinck |first7=Mark |date=2021-03-26 |title=Wave dark matter and ultra-diffuse galaxies |journal=Monthly Notices of the Royal Astronomical Society |volume=504 |issue=2 |pages=2868–2876 |doi=10.1093/mnras/stab855 |doi-access=free |issn=0035-8711|arxiv=2003.08313 }} 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.{{cite news |last1=Crosswell |first1=Ken |title=The Milky Way's dark twin revealed |url=http://www.nature.com/news/the-milky-way-s-dark-twin-revealed-1.20333 |access-date=30 July 2016 |publisher=Nature News |date=26 July 2016}} On 25 August 2016, astronomers reported that Dragonfly 44 may be made almost entirely of dark matter.{{cite journal |author1=van Dokkum, Pieter |display-authors=etal |title=A high stellar velocity dispersion and ~100 globular clusters for the ultra-diffuse galaxy Dragonfly 44 |date=25 August 2016 |journal=The Astrophysical Journal Letters |doi=10.3847/2041-8205/828/1/L6 |volume=828 |issue=1 |pages=L6 |arxiv=1606.06291 |bibcode=2016ApJ...828L...6V |s2cid=1275440 |url=http://scholarworks.sjsu.edu/physics_astron_pub/174 |doi-access=free }}{{cite web |last=Hall |first=Shannon |title=Ghost galaxy is 99.99 per cent dark matter with almost no stars |url=https://www.newscientist.com/article/2102584-ghost-galaxy-is-99-99-per-cent-dark-matter-with-almost-no-stars/ |date=25 August 2016 |work=New Scientist |access-date=27 August 2016}}{{cite news |last=Feltman |first=Rachael |title=A new class of galaxy has been discovered, one made almost entirely of dark matter |url=https://www.washingtonpost.com/news/speaking-of-science/wp/2016/08/25/a-new-class-of-galaxy-has-been-discovered-one-made-almost-entirely-of-dark-matter |date=26 August 2016 |newspaper=The Washington Post |access-date=26 August 2016}} 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.{{Cite journal|title=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|journal=The Astrophysical Journal|year=2019|language=en|doi=10.3847/1538-4357/ab2914 |arxiv=1904.04838|last1=Van Dokkum|first1=Pieter|last2=Wasserman|first2=Asher|last3=Danieli|first3=Shany|last4=Abraham|first4=Roberto|last5=Brodie|first5=Jean|last6=Conroy|first6=Charlie|last7=Forbes|first7=Duncan A.|last8=Martin|first8=Christopher|last9=Matuszewski|first9=Matt|last10=Romanowsky|first10=Aaron J.|last11=Villaume|first11=Alexa|volume=880|issue=2|page=91|s2cid=119185644 |doi-access=free }} Dragonfly 44 is one of the most metal-poor ultra diffuse galaxies known, with measurements of globular clusters in the Milky Way and M87 galaxies showing that stellar populations in the inner core of Dragonfly 44 are similar to metal-poor globular clusters.{{Cite journal |last1=Villaume |first1=Alexa |last2=Romanowsky |first2=Aaron J. |last3=Brodie |first3=Jean |last4=van Dokkum |first4=Pieter |last5=Conroy |first5=Charlie |last6=Forbes |first6=Duncan A. |last7=Danieli |first7=Shany |last8=Martin |first8=Christopher |last9=Matuszewski |first9=Matt |date=2022-01-01 |title=Spatially Resolved Stellar Spectroscopy of the Ultra-diffuse Galaxy Dragonfly 44. III. Evidence for an Unexpected Star Formation History under Conventional Galaxy Evolution Processes |journal=The Astrophysical Journal |volume=924 |issue=1 |pages=32 |doi=10.3847/1538-4357/ac341e |doi-access=free |arxiv=2101.02220 |bibcode=2022ApJ...924...32V |issn=0004-637X}} The most recent work found 20 globular clusters around the galaxy, which is consistent with the recent mass measurement.{{Cite journal|title="The number of globular clusters around the iconic UDG DF44 is as expected for dwarf galaxies", Saifollahi et al. 2020|journal=Monthly Notices of the Royal Astronomical Society|doi = 10.1093/mnras/staa3016| arxiv=2006.14630 |doi-access=free|last1=Saifollahi |first1=Teymoor |last2=Trujillo |first2=Ignacio |last3=Beasley |first3=Michael A. |last4=Peletier |first4=Reynier F. |last5=Knapen |first5=Johan H. |year=2020 }}{{Cite news|title=The puzzle of the strange galaxy made of 99.99% dark matter is solved|url=https://www.iac.es/en/outreach/news/puzzle-strange-galaxy-made-9999-dark-matter-solved}} 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.{{Cite journal |last=Bogdán |first=Ákos |year=2020 |title=The Archetypal Ultra-diffuse Galaxy, Dragonfly 44, Is not a Dark Milky Way |journal=The Astrophysical Journal Letters |volume=901 |issue=2 |pages=L30 |arxiv=2009.07846 |doi=10.3847/2041-8213/abb886 |issn=2041-8205 |s2cid=221761669 |doi-access=free|bibcode=2020ApJ...901L..30B }}

In 2018, the same authors reported the discovery that the ultra diffuse galaxy NGC 1052-DF2{{efn|NGC 1052-DF2 had previously been identified on photographic plates by Karachentsev et al. in 2000.{{cite journal |author1=Karachentsev, I.D. |author1-link=Igor Karachentsev |author2=Karachentseva, V.E. |author3=Suchkov, A.A. |author4=Grebel, E.K. |title=Dwarf galaxy candidates found on the SERC EJ sky survey |journal=Astron. Astrophys. Suppl. Ser. |volume=145 |pages=415–423 |year=2000|issue=3 |doi=10.1051/aas:2000249 |bibcode=2000A&AS..145..415K |doi-access=free }}}} is dark matter-free, based on velocity measurements of its ~10 globular cluster system.{{cite news |url=http://blogs.discovermagazine.com/d-brief/2019/10/21/hubble-reveals-new-evidence-for-controversial-galaxies-without-dark-matter/ |title=Hubble reveals new evidence for controversial galaxies without dark matter |author=Parks, Jake |website=Discover Magazine |date=2019-10-21 |df=dmy-all}}{{cite journal |author1=van Dokkum |display-authors=etal |year=2018 |journal=Nature |volume=555 |pages=629–632 |title=A galaxy lacking dark matter|issue=7698 |doi=10.1038/nature25767 |pmid=29595770 |arxiv=1803.10237 |bibcode=2018Natur.555..629V |s2cid=4460376 }} They concluded that this may rule out some alternative gravity theories like modified Newtonian dynamics (MOND), unless one takes its external field effect into account. Detailed simulations in the framework of MOND confirm that NGC 1052-DF2 is quite consistent with theoretical expectations.{{Cite journal|last1=Haghi|first1=Hosein|last2=Kroupa|first2=Pavel|last3=Banik|first3=Indranil|last4=Wu|first4=Xufen|last5=Zonoozi|first5=Akram H.|last6=Javanmardi|first6=Behnam|last7=Ghari|first7=Amir|last8=Müller|first8=Oliver|last9=Dabringhausen|first9=Jörg|last10=Zhao|first10=Hongsheng|date=2019-08-14|title=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|journal=Monthly Notices of the Royal Astronomical Society|volume=487|issue=2|pages=2441–2454|doi=10.1093/mnras/stz1465|arxiv=1906.03268|bibcode=2019MNRAS.487.2441H|issn=0035-8711|doi-access=free}}

In 2021, AGC 114905, an ultra-diffuse dwarf galaxy about 250 million light-years away, was reported to have almost no dark matter.{{Cite web |date=December 6, 2021 |title=Evidence emerges for dark-matter free galaxies |url=https://phys.org/news/2021-12-evidence-emerges-dark-matter-free-galaxies.html |access-date=2024-09-21 |website=Phys.org |language=en}} 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.{{Cite web |date=19 May 2022 |title=New spin on galaxy rotation saves controversial gravity theory |url=https://news.st-andrews.ac.uk/archive/new-spin-on-galaxy-rotation-saves-controversial-gravity-theory/ |access-date=2024-09-21 |website=University of St Andrews |language=en-US}} 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.{{Cite journal |last1=Sellwood |first1=J A |last2=Sanders |first2=R H |date=2022-06-28 |title=The ultradiffuse galaxy AGC 114905 needs dark matter |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=514 |issue=3 |pages=4008–4017 |arxiv=2202.08678 |doi=10.1093/mnras/stac1604 |doi-access=free |issn=0035-8711}}

Ultra diffuse galaxy

Examples

See also

Footnotes

References

Further reading