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| {{Short description|A hypothesized galaxy with no, or very few, stars}} | {{Short description|A hypothesized galaxy with no, or very few, stars}} | ||
| A '''dark galaxy''' is a hypothesized ] with no (or very few) stars. They received their name because they have no visible stars |
A '''dark galaxy''' is a hypothesized ] 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.<ref>{{Cite journal|title = The Alfalfa "Almost Darks" Campaign: Pilot VLA HI Observations of Five High Mass-To-Light Ratio Systems|journal = The Astronomical Journal|date = 2015-02-01|issn = 0004-6256|pages = 72|volume = 149|issue = 2|doi = 10.1088/0004-6256/149/2/72|first1 = John M.|last1 = Cannon|first2 = Charlotte P.|last2 = Martinkus|first3 = Lukas|last3 = Leisman|first4 = Martha P.|last4 = Haynes|first5 = Elizabeth A. K.|last5 = Adams|first6 = Riccardo|last6 = Giovanelli|first7 = Gregory|last7 = Hallenbeck|first8 = Steven|last8 = Janowiecki|first9 = Michael|last9 = Jones|arxiv = 1412.3018 |bibcode = 2015AJ....149...72C |s2cid = 118592858}}</ref> Dark galaxies are distinct from intergalactic ]s caused by ], since these gas clouds do not contain ], 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.<ref>{{Cite journal|title = Is GBT 1355+5439 a dark galaxy?|journal = Astronomy & Astrophysics|volume = 555|pages = L7|doi = 10.1051/0004-6361/201321965|year = 2013|last1 = Oosterloo|first1 = T. A.|last2 = Heald|first2 = G. H.|last3 = De Blok|first3 = W. J. G.|bibcode = 2013A&A...555L...7O|arxiv = 1306.6148|s2cid = 118402005}}</ref> The best candidate dark galaxies to date{{As of when|date=May 2024}} include HI1225+01,<ref>{{Cite web|title = The Arecibo Legacy Fast ALFA Survey|url = http://egg.astro.cornell.edu/alfalfa/HI1225+01.php|website = egg.astro.cornell.edu|access-date = 2015-09-29}}</ref> AGC229385,<ref>{{Cite journal|title = (Almost) Dark HI Sources in the ALFALFA Survey: The Intriguing Case of HI1232+20|journal = The Astrophysical Journal|date = 2015-03-01|issn = 0004-637X|pages = 96|volume = 801|issue = 2|doi = 10.1088/0004-637X/801/2/96|first1 = Steven|last1 = Janowiecki|first2 = Lukas|last2 = Leisman|first3 = Gyula|last3 = Józsa|first4 = John J.|last4 = Salzer|first5 = Martha P.|last5 = Haynes|first6 = Riccardo|last6 = Giovanelli|first7 = Katherine L.|last7 = Rhode|first8 = John M.|last8 = Cannon|first9 = Elizabeth A. K.|last9 = Adams|arxiv = 1502.01296 |bibcode = 2015ApJ...801...96J |s2cid = 119271121}}</ref> and numerous gas clouds detected in studies of ]s. | ||
| On 25 August 2016, astronomers reported that ], an ] (UDG) with the ] of the ] ], but with nearly no |
On 25 August 2016, astronomers reported that ], an ] (UDG) with the ] of the ] ], but with nearly no discernible ]s or galactic structure, is made almost entirely of ].<ref name="AJL-20160825">{{cite journal |author=Van Dokkum, Pieter|display-authors=et al |title=A High Stellar Velocity Dispersion and ~100 Globular Clusters For The Ultra-Diffuse Galaxy Dragonfly 44 |date=25 August 2016 |journal=] |doi=10.3847/2041-8205/828/1/L6 |volume=828 |issue=1 |page=L6|arxiv = 1606.06291 |bibcode = 2016ApJ...828L...6V |s2cid=1275440 |url=http://scholarworks.sjsu.edu/physics_astron_pub/174 |doi-access=free }}</ref><ref name="NS-20160825">{{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=] |access-date=27 August 2016 }}</ref><ref name="WP-20160826">{{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=] |access-date=26 August 2016}}</ref> | ||
| ==Observational evidence== | ==Observational evidence== | ||
| ⚫ | ] with sensitive but low-resolution ]s like ] or the ] look for ] from ] in galaxies. These surveys are then matched to optical surveys to identify any objects with no optical counterpart—that is, sources with no stars.<ref>{{Cite journal |title=The existence and detection of optically dark galaxies by 21-cm surveys |journal=Monthly Notices of the Royal Astronomical Society |date=2006 |doi=10.1111/j.1365-2966.2006.10247.x |last1=Davies |first1=J. I. |last2=Disney |first2=M. J. |last3=Minchin |first3=R. F. |last4=Auld |first4=R. |last5=Smith |first5=R. |volume=368 |issue=3 |pages=1479–1488 |doi-access=free |arxiv=astro-ph/0609747 |bibcode=2006MNRAS.368.1479D }}</ref> | ||
| {{unreferenced |section|date=March 2023}} | |||
| ⚫ | ] with sensitive but low-resolution ]s like ] or the ] look for ] from ] in galaxies. These surveys are then matched to optical surveys to identify any objects with no optical |
||
| Another way astronomers search for dark galaxies is to look for ]s in the ] of background ]s. This technique has revealed many intergalactic clouds of hydrogen, but following up on candidate dark galaxies is difficult, since these sources tend to be too far away and are often optically drowned out by the bright light from the quasars. | Another way astronomers search for dark galaxies is to look for ]s in the ] of background ]s. This technique has revealed many intergalactic clouds of hydrogen, but following up on candidate dark galaxies is difficult, since these sources tend to be too far away and are often optically drowned out by the bright light from the quasars. | ||
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| ===Structure=== | ===Structure=== | ||
| Dark galaxies are theoretically composed of ], hydrogen and ].<ref name="universetoday" /> Some scientists support the idea that dark galaxies may contain stars.<ref name="newscientist">{{cite magazine |url=https://www.newscientist.com/article/dn12100-dark-galaxy-continues-to-puzzle-astronomers.html |title=Dark galaxy' continues to puzzle astronomers |access-date=2008-02-26 |magazine=]|author=Stuart Clark|author-link=Stuart Clark (author)}}</ref> Yet the exact composition of dark galaxies remains unknown because there is no conclusive way to identify them. Nevertheless, astronomers estimate that the mass of the gas in these galaxies is approximately one |
Dark galaxies are theoretically composed of ], hydrogen, and ].<ref name="universetoday" /> Some scientists support the idea that dark galaxies may contain stars.<ref name="newscientist">{{cite magazine |url=https://www.newscientist.com/article/dn12100-dark-galaxy-continues-to-puzzle-astronomers.html |title=Dark galaxy' continues to puzzle astronomers |access-date=2008-02-26 |magazine=]|author=Stuart Clark|author-link=Stuart Clark (author)}}</ref> Yet the exact composition of dark galaxies remains unknown because there is no conclusive way to identify them. Nevertheless, astronomers estimate that the mass of the gas in these galaxies is approximately one billion times ].<ref name="FirstDirectDetection">{{cite web|url=https://www.sciencedaily.com/releases/2012/10/121030161234.htm |title=First Direct Detection Sheds Light On Dark Galaxies |publisher=Zmescience.com |access-date=22 December 2012}}</ref> | ||
| ===Methodology to observe dark bodies=== | ===Methodology to observe dark bodies=== | ||
| Dark galaxies contain no visible stars and are invisible to optical telescopes. The Arecibo Galaxy Environment Survey (AGES) harnessed the ] to search for dark galaxies, which are predicted to contain detectable amounts of ]. The Arecibo radio telescope was useful where others are not because of its ability to detect the emission from this neutral hydrogen, specifically the 21-cm line. | Dark galaxies contain no visible stars and are invisible to optical telescopes. The Arecibo Galaxy Environment Survey (AGES) harnessed the ] to search for dark galaxies, which are predicted to contain detectable amounts of ]. The Arecibo radio telescope was useful where others are not because of its ability to detect the emission from this neutral hydrogen, specifically the ]. | ||
| ===Alternative theories=== | ===Alternative theories=== | ||
| Scientists say that the galaxies we see today only began to create stars after dark galaxies. Based on numerous scientific assertions, dark galaxies played a big role in many of the galaxies astronomers and scientists see today. Martin Haehnel, from Kavli Institute for Cosmology at the University of Cambridge, claims that the precursor to the Milky Way galaxy was actually a much smaller bright galaxy that had merged with dark galaxies nearby to form the Milky Way we currently see. Multiple scientists agree that dark galaxies are building blocks of modern galaxies. Sebastian Cantalupo of the University of California, Santa Cruz, agrees with this theory. He goes on to say, "In our current theory of galaxy formation, we believe that big galaxies form from the merger of smaller galaxies. Dark galaxies bring to big galaxies a lot of gas, which then accelerates star formation in the bigger galaxies." Scientists have specific techniques they use to locate these dark galaxies. These techniques have the capability of teaching us more about other special events that occur in the universe; for instance, the |
Scientists say that the galaxies we see today only began to create stars after dark galaxies. Based on numerous scientific assertions, dark galaxies played a big role in many of the galaxies astronomers and scientists see today. Martin Haehnel, from ] at the ], claims that the precursor to the ] galaxy was actually a much smaller bright galaxy that had merged with dark galaxies nearby to form the Milky Way we currently see. Multiple scientists agree that dark galaxies are building blocks of modern galaxies. Sebastian Cantalupo of the ], agrees with this theory. He goes on to say, "In our current theory of galaxy formation, we believe that big galaxies form from the merger of smaller galaxies. Dark galaxies bring to big galaxies a lot of gas, which then accelerates star formation in the bigger galaxies." Scientists have specific techniques they use to locate these dark galaxies. These techniques have the capability of teaching us more about other special events that occur in the universe; for instance, the ]. This "web" is made of invisible filaments of gas and dark matter believed to permeate the universe, as well as "feeding and building galaxies and galaxy clusters where the filaments intersect."<ref name="FirstDirectDetection"/> | ||
| ==Potential dark galaxies== | ==Potential dark galaxies== | ||
| ===FAST J0139+4328=== | ===FAST J0139+4328=== | ||
| {{main|FAST J0139+4328}} | |||
| Located 94 million light years away from Earth, this galaxy is visible in radio waves with minimal visible light.<ref name="mysterious-galaxy">{{cite web | url=https://www.sciencealert.com/mysterious-dark-galaxy-emits-no-visible-light-scientists-say | title=Mysterious Dark Galaxy Emits No Visible Light, Scientists Say | date=10 February 2023 }}</ref> | Located 94 million light years away from Earth, this galaxy is visible in radio waves with minimal visible light.<ref name="mysterious-galaxy">{{cite web | url=https://www.sciencealert.com/mysterious-dark-galaxy-emits-no-visible-light-scientists-say | title=Mysterious Dark Galaxy Emits No Visible Light, Scientists Say | date=10 February 2023 }}</ref> | ||
| ===HE0450-2958=== | ===HE0450-2958=== | ||
| {{main|HE0450-2958}} | {{main|HE0450-2958}} | ||
| ⚫ | HE0450-2958 is a ] at ] z=0.285. ] images showed that the quasar is located at the edge of a large cloud of gas, but no host galaxy was detected for the quasar. The authors of the Hubble study suggested that one possible scenario was that the quasar is located in a dark galaxy.<ref>{{cite journal | author = Magain P. | display-authors = etal | year = 2005 | title = Discovery of a bright quasar without a massive host galaxy | bibcode = 2005Natur.437..381M | journal = Nature | volume = 437 | issue = 7057| pages = 381–4 | doi=10.1038/nature04013|arxiv = astro-ph/0509433 | pmid=16163349| s2cid = 4303895 }}</ref> However, subsequent analysis by other groups found no evidence that the host galaxy is anomalously dark, and demonstrated that a normal host galaxy is probably present,<ref>{{cite journal | last = Merritt | first = David | display-authors = etal | author-link=David Merritt | year = 2006 | title = The nature of the HE0450-2958 system | bibcode = 2006MNRAS.367.1746M | journal = ]| volume = 367 | issue = 4| pages = 1746–1750 | doi=10.1111/j.1365-2966.2006.10093.x| doi-access = free |arxiv = astro-ph/0511315 | s2cid = 55834970 }}</ref><ref>{{cite journal |last1=Kim |first1=Minjin |last2=Ho |first2=Luis |last3=Peng |first3=Chien |last4=Im |first4=Myungshin |title=The Host Galaxy of the Quasar HE 0450-2958 |journal=The Astrophysical Journal |date=2007 |volume=658 |issue=1 |pages=107–113 |doi=10.1086/510846 |bibcode=2007ApJ...658..107K|arxiv=astro-ph/0611411 |s2cid=14375599 }}</ref> so the observations do not support the dark galaxy interpretation. | ||
| ⚫ | HE0450-2958 is a ] at ] z=0.285. ] images showed that the quasar is located at the edge of a large cloud of gas, but no host galaxy was detected for the quasar. The authors of the Hubble study suggested that one possible scenario was that the quasar is located in a dark galaxy.<ref>{{cite journal | author = Magain P. | display-authors = etal | year = 2005 | title = Discovery of a bright quasar without a massive host galaxy | bibcode = 2005Natur.437..381M | journal = Nature | volume = 437 | issue = 7057| pages = 381–4 | doi=10.1038/nature04013|arxiv = astro-ph/0509433 | pmid=16163349| s2cid = 4303895 }}</ref> However, subsequent analysis by other groups found no evidence that the host galaxy is anomalously dark, and demonstrated that a normal host galaxy is probably present,<ref>{{cite journal | last = Merritt | first = David | display-authors = etal | author-link=David Merritt | year = 2006 | title = The nature of the HE0450-2958 system | bibcode = 2006MNRAS.367.1746M | journal = ]| volume = 367 | issue = 4| pages = 1746–1750 | doi=10.1111/j.1365-2966.2006.10093.x|arxiv = astro-ph/0511315 | s2cid = 55834970 }}</ref><ref>{{cite journal |last1=Kim |first1=Minjin |last2=Ho |first2=Luis |last3=Peng |first3=Chien |last4=Im |first4=Myungshin |title=The Host Galaxy of the Quasar HE 0450-2958 |journal=The Astrophysical Journal |date=2007 |volume=658 |issue=1 |pages=107–113 |doi=10.1086/510846 |bibcode=2007ApJ...658..107K|arxiv=astro-ph/0611411 |s2cid=14375599 }}</ref> so the observations do not support the dark galaxy interpretation. | ||
| ===HVC 127-41-330=== | ===HVC 127-41-330=== | ||
| {{main|HVC 127-41-330}} | {{main|HVC 127-41-330}} | ||
| HVC 127-41-330 is a cloud rotating at high speed between ] and the ]. Astronomer Josh Simon considers this cloud to be a dark galaxy because of the speed of its rotation and its predicted mass.<ref name="simon">{{cite |
HVC 127-41-330 is a cloud rotating at high speed between ] and the ]. Astronomer Josh Simon considers this cloud to be a dark galaxy because of the speed of its rotation and its predicted mass.<ref name="simon">{{cite thesis |first = Josh |last = Simon | title = Dark Matter in Dwarf Galaxies: Observational Tests of the Cold Dark Matter Paradigm on Small Scales | degree = PhD | year = 2005 | url = http://www.astro.caltech.edu/~jsimon/thesis/jdsthesis.pdf|archive-url=https://web.archive.org/web/20060913084945/http://www.astro.caltech.edu/~jsimon/thesis/jdsthesis.pdf|archive-date=2006-09-13| bibcode = 2005PhDT.........2S |institution = ]}}</ref><ref>{{cite news| last =Battersby | first =Stephen | title =Astronomers find first 'dark galaxy' | work =New Scientist | url = https://www.newscientist.com/article.ns?id=dn4272 | date =2003-10-20 |access-date=December 22, 2012}}</ref> | ||
| ===J0613+52=== | |||
| J0613+52 is a possible dark galaxy,<ref name="NYT-20240126">{{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=] |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 }}</ref><ref>{{Cite journal |last1=O'Neil |first1=Karen |last2=van Driel |first2=Wim |last3=Schneider |first3=Stephen |date=2024-02-01 |title=The Dark Galaxy J0613+52 |url=https://ui.adsabs.harvard.edu/abs/2024AAS...24340413O |journal=Bulletin of the American Astronomical Society |volume=56 |issue=2 |pages=404.13 |bibcode=2024AAS...24340413O}}</ref> discovered with the ] when it was accidentally pointed to the wrong ]. Stars could possibly exist within it, but were observed as of January 2024.<ref>{{Cite web |last=Malusky |first=Jill |date=2024-01-08 |title=Astronomers Accidentally Discover Dark Primordial Galaxy |url=https://greenbankobservatory.org/news/astronomers-accidentally-discover-dark-primordial-galaxy/ |archive-url=https://web.archive.org/web/20240309075731/https://greenbankobservatory.org/news/astronomers-accidentally-discover-dark-primordial-galaxy/ |archive-date=2024-03-09 |access-date=2024-03-10 |website=Green Bank Observatory}}</ref> | |||
| ===Nube=== | |||
| Nube was discovered in 2023 by analyzing deep optical imagery of an area in ]. Due to its low surface brightness, Nube is classified as an "almost dark galaxy."<ref>{{Cite web|title=Astronomers discover new almost dark galaxy|url=https://phys.org/news/2023-10-astronomers-dark-galaxy.html|access-date=2024-02-06|website=Phys.org|language=en}}</ref><ref>{{Cite journal |last1=Montes |first1=Mireia |last2=Trujillo |first2=Ignacio |last3=Karunakaran |first3=Ananthan |last4=Infante-Sainz |first4=Raúl |last5=Spekkens |first5=Kristine |last6=Golini |first6=Giulia |last7=Beasley |first7=Michael |last8=Cebrián |first8=Maria |last9=Chamba |first9=Nushkia |last10=D’Onofrio |first10=Mauro |last11=Kelvin |first11=Lee |last12=Román |first12=Javier |date=January 2024 |title=An almost dark galaxy with the mass of the Small Magellanic Cloud |url=https://www.aanda.org/10.1051/0004-6361/202347667 |journal=Astronomy & Astrophysics |volume=681 |pages=A15 |doi=10.1051/0004-6361/202347667 |issn=0004-6361|arxiv=2310.12231 |bibcode=2024A&A...681A..15M }}</ref> | |||
| ===Smith's Cloud=== | ===Smith's Cloud=== | ||
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| ===VIRGOHI21=== | ===VIRGOHI21=== | ||
| {{main|VIRGOHI21}} | {{main|VIRGOHI21}} | ||
| ⚫ | Initially discovered in 2000, VIRGOHI21 was announced in February 2005 as a good candidate to be a true dark galaxy.<ref name=newscientist/><ref name="firstDark">{{cite news| last =Clark | first =Stuart | title =Astronomers claim first 'dark galaxy' find | work =NewScientist.com news service | url =https://www.newscientist.com/article.ns?id=dn7056 | date=2005-02-23| access-date =2006-10-26 }}</ref><ref>{{cite journal | last =Shiga | first =David | title =Ghostly Galaxy: Massive, dark cloud intrigues scientists | journal =Science News Online | volume =167 | issue =9 | pages =131 | date =2005-02-26 | url =http://www.sciencenews.org/view/generic/id/5929/title/Ghostly_Galaxy_Massive%2C_dark_cloud_intrigues_scientists | access-date =2008-09-14 | doi =10.2307/4015891 | jstor =4015891 | archive-url =https://web.archive.org/web/20110524102600/http://www.sciencenews.org/view/generic/id/5929/title/Ghostly_Galaxy_Massive,_dark_cloud_intrigues_scientists | archive-date =2011-05-24 | url-status =dead }}</ref><ref>{{cite news| last =Britt | first =Roy | title =First Invisible Galaxy Discovered in Cosmology Breakthrough | work =Space.com | url = http://www.space.com/scienceastronomy/050223_dark_galaxy.html | date =2005-02-23 }}</ref> It was detected in ] surveys, and was suspected to be a possible cosmic partner to the galaxy ]. This unusual-looking galaxy appears to be one partner in a cosmic collision, and appeared to show dynamics consistent with a dark galaxy (and apparently inconsistent with the predictions of the ] theory).<ref>{{cite journal| last =Funkhouser| first =Scott| title =Testing MOND with VirgoHI21 | journal =Monthly Notices of the Royal Astronomical Society | volume =364| issue =1| page =237 | year =2005 | arxiv = astro-ph/0503104 | doi =10.1111/j.1365-2966.2005.09565.x | doi-access =free| bibcode=2005MNRAS.364..237F| s2cid =119368923}}</ref> However, further observations revealed that VIRGOHI21 was an intergalactic gas cloud, stripped from NGC4254 by a high speed collision.<ref>{{Cite journal|title = Optically Unseen H I Detections toward the Virgo Cluster Detected in the Arecibo Legacy Fast ALFA Survey|journal = The Astrophysical Journal Letters|date = 2007-08-01|issn = 0004-637X|pages = L15–L18|volume = 665|issue = 1|doi = 10.1086/521100|first1 = Brian R.|last1 = Kent|first2 = Riccardo|last2 = Giovanelli|first3 = Martha P.|last3 = Haynes|first4 = Amélie|last4 = Saintonge|first5 = Sabrina|last5 = Stierwalt|first6 = Thomas|last6 = Balonek|first7 = Noah|last7 = Brosch|first8 = Barbara|last8 = Catinella|first9 = Rebecca A.|last9 = Koopmann|bibcode = 2007ApJ...665L..15K |doi-access = free}}</ref><ref>{{Cite journal|title = Tidal Debris from High-Velocity Collisions as Fake Dark Galaxies: A Numerical Model of VIRGOHI 21|journal = The Astrophysical Journal|date = 2008-02-01|issn = 0004-637X|pages = 787–797|volume = 673|issue = 2|doi = 10.1086/524868|first1 = Pierre-Alain|last1 = Duc|first2 = Frederic|last2 = Bournaud|arxiv = 0710.3867 |bibcode = 2008ApJ...673..787D |s2cid = 15348867}}</ref><ref>{{cite journal |author1=Haynes, Martha P. |author2=Giovanelli, Riccardo |author3=Kent, Brian R. | title= NGC 4254: An Act of Harassment Uncovered by the Arecibo Legacy Fast ALFA Survey | journal=] | year=2007 | volume=665 | issue=1 | pages=L19–22 | doi=10.1086/521188 | bibcode=2007ApJ...665L..19H|arxiv = 0707.0113 |s2cid=12930657 }}</ref> The high speed interaction was caused by infall into the ]. | ||
| ⚫ | Initially discovered in 2000, VIRGOHI21 was announced in February 2005 as a good candidate to be a true dark galaxy.<ref name=newscientist/><ref name="firstDark">{{cite news| last =Clark | first =Stuart | title =Astronomers claim first 'dark galaxy' find | work =NewScientist.com news service | url =https://www.newscientist.com/article.ns?id=dn7056 | date=2005-02-23| access-date =2006-10-26 }}</ref><ref>{{cite journal | last =Shiga | first =David | title =Ghostly Galaxy: Massive, dark cloud intrigues scientists | journal =Science News Online | volume =167 | issue =9 | pages =131 | date =2005-02-26 | url =http://www.sciencenews.org/view/generic/id/5929/title/Ghostly_Galaxy_Massive%2C_dark_cloud_intrigues_scientists | access-date =2008-09-14 | doi =10.2307/4015891 | jstor =4015891 | archive-url =https://web.archive.org/web/20110524102600/http://www.sciencenews.org/view/generic/id/5929/title/Ghostly_Galaxy_Massive,_dark_cloud_intrigues_scientists | archive-date =2011-05-24 | url-status =dead }}</ref><ref>{{cite news| last =Britt | first =Roy | title =First Invisible Galaxy Discovered in Cosmology Breakthrough | work =Space.com | url = http://www.space.com/scienceastronomy/050223_dark_galaxy.html | date =2005-02-23 }}</ref> It was detected in 21-cm surveys, and was suspected to be a possible cosmic partner to the galaxy ]. This unusual-looking galaxy appears to be one partner in a cosmic collision, and appeared to show dynamics consistent with a dark galaxy (and apparently inconsistent with the predictions of the ] theory).<ref>{{cite journal| last =Funkhouser| first =Scott| title =Testing MOND with VirgoHI21 | journal =Monthly Notices of the Royal Astronomical Society | volume =364| issue =1| page =237 | year =2005 | arxiv = astro-ph/0503104 | doi =10.1111/j.1365-2966.2005.09565.x | bibcode=2005MNRAS.364..237F| s2cid =119368923}}</ref> However, further observations revealed that VIRGOHI21 was an intergalactic gas cloud, stripped from NGC4254 by a high speed collision.<ref>{{Cite journal|title = Optically Unseen H I Detections toward the Virgo Cluster Detected in the Arecibo Legacy Fast ALFA Survey|journal = The Astrophysical Journal Letters|date = 2007-08-01|issn = 0004-637X|pages = L15–L18|volume = 665|issue = 1|doi = 10.1086/521100|first1 = Brian R.|last1 = Kent|first2 = Riccardo|last2 = Giovanelli|first3 = Martha P.|last3 = Haynes|first4 = Amélie|last4 = Saintonge|first5 = Sabrina|last5 = Stierwalt|first6 = Thomas|last6 = Balonek|first7 = Noah|last7 = Brosch|first8 = Barbara|last8 = Catinella|first9 = Rebecca A.|last9 = Koopmann|bibcode = 2007ApJ...665L..15K |doi-access = free}}</ref><ref>{{Cite journal|title = Tidal Debris from High-Velocity Collisions as Fake Dark Galaxies: A Numerical Model of VIRGOHI 21|journal = The Astrophysical Journal|date = 2008-02-01|issn = 0004-637X|pages = 787–797|volume = 673|issue = 2|doi = 10.1086/524868|first1 = Pierre-Alain|last1 = Duc|first2 = Frederic|last2 = Bournaud|arxiv = 0710.3867 |bibcode = 2008ApJ...673..787D |s2cid = 15348867}}</ref><ref>{{cite journal |author1=Haynes, Martha P. |author2=Giovanelli, Riccardo |author3=Kent, Brian R. | title= NGC 4254: An Act of Harassment Uncovered by the Arecibo Legacy Fast ALFA Survey | journal=] | year=2007 | volume=665 | issue=1 | pages=L19–22 | doi=10.1086/521188 | bibcode=2007ApJ...665L..19H|arxiv = 0707.0113 |s2cid=12930657 }}</ref> The high speed interaction was caused by infall into the ]. | ||
| ==See also== | ==See also== | ||
| Line 58: | Line 60: | ||
| * {{annotated link|Low-surface-brightness galaxy}} | * {{annotated link|Low-surface-brightness galaxy}} | ||
| * {{annotated link|Ultra diffuse galaxy}} | * {{annotated link|Ultra diffuse galaxy}} | ||
| * ] – the most dark matter-dominated systems known | |||
| * {{annotated link|Galaxy X (galaxy)}} | * {{annotated link|Galaxy X (galaxy)}} | ||
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Latest revision as of 06:23, 9 October 2024
A hypothesized galaxy with no, or very few, starsA 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 discernible stars or galactic structure, is made almost entirely of dark matter.
Observational evidence
Large surveys with sensitive but low-resolution radio telescopes like Arecibo or the Parkes Telescope look for 21-cm emission from atomic hydrogen in galaxies. These surveys are then matched to optical surveys to identify any objects with no optical counterpart—that is, sources with no stars.
Another way astronomers search for dark galaxies is to look for hydrogen absorption lines in the spectra of background quasars. This technique has revealed many intergalactic clouds of hydrogen, but following up on candidate dark galaxies is difficult, since these sources tend to be too far away and are often optically drowned out by the bright light from the quasars.
Nature of dark galaxies
Origin
In 2005, astronomers discovered gas cloud VIRGOHI21 and attempted to determine what it was and why it exerted such a massive gravitational pull on galaxy NGC 4254. After years of ruling out other possible explanations, some have concluded that VIRGOHI21 is a dark galaxy.
Size
The actual size of dark galaxies is unknown because they cannot be observed with normal telescopes. There have been various estimations, ranging from double the size of the Milky Way to the size of a small quasar.
Structure
Dark galaxies are theoretically composed of dark matter, hydrogen, and dust. Some scientists support the idea that dark galaxies may contain stars. Yet the exact composition of dark galaxies remains unknown because there is no conclusive way to identify them. Nevertheless, astronomers estimate that the mass of the gas in these galaxies is approximately one billion times that of the Sun.
Methodology to observe dark bodies
Dark galaxies contain no visible stars and are invisible to optical telescopes. The Arecibo Galaxy Environment Survey (AGES) harnessed the Arecibo radio telescope to search for dark galaxies, which are predicted to contain detectable amounts of neutral hydrogen. The Arecibo radio telescope was useful where others are not because of its ability to detect the emission from this neutral hydrogen, specifically the 21-cm line.
Alternative theories
Scientists say that the galaxies we see today only began to create stars after dark galaxies. Based on numerous scientific assertions, dark galaxies played a big role in many of the galaxies astronomers and scientists see today. Martin Haehnel, from Kavli Institute for Cosmology at the University of Cambridge, claims that the precursor to the Milky Way galaxy was actually a much smaller bright galaxy that had merged with dark galaxies nearby to form the Milky Way we currently see. Multiple scientists agree that dark galaxies are building blocks of modern galaxies. Sebastian Cantalupo of the University of California, Santa Cruz, agrees with this theory. He goes on to say, "In our current theory of galaxy formation, we believe that big galaxies form from the merger of smaller galaxies. Dark galaxies bring to big galaxies a lot of gas, which then accelerates star formation in the bigger galaxies." Scientists have specific techniques they use to locate these dark galaxies. These techniques have the capability of teaching us more about other special events that occur in the universe; for instance, the cosmic web. This "web" is made of invisible filaments of gas and dark matter believed to permeate the universe, as well as "feeding and building galaxies and galaxy clusters where the filaments intersect."
Potential dark galaxies
FAST J0139+4328
Located 94 million light years away from Earth, this galaxy is visible in radio waves with minimal visible light.
HE0450-2958
Main article: HE0450-2958HE0450-2958 is a quasar at redshift z=0.285. Hubble Space Telescope images showed that the quasar is located at the edge of a large cloud of gas, but no host galaxy was detected for the quasar. The authors of the Hubble study suggested that one possible scenario was that the quasar is located in a dark galaxy. However, subsequent analysis by other groups found no evidence that the host galaxy is anomalously dark, and demonstrated that a normal host galaxy is probably present, so the observations do not support the dark galaxy interpretation.
HVC 127-41-330
Main article: HVC 127-41-330HVC 127-41-330 is a cloud rotating at high speed between Andromeda and the Triangulum Galaxy. Astronomer Josh Simon considers this cloud to be a dark galaxy because of the speed of its rotation and its predicted mass.
J0613+52
J0613+52 is a possible dark galaxy, discovered with the Green Bank Telescope when it was accidentally pointed to the wrong coordinates. Stars could possibly exist within it, but were observed as of January 2024.
Nube
Nube was discovered in 2023 by analyzing deep optical imagery of an area in Stripe 82. Due to its low surface brightness, Nube is classified as an "almost dark galaxy."
Smith's Cloud
Main article: Smith's CloudSmith's Cloud is a candidate to be a dark galaxy, due to its projected mass and survival of encounters with the Milky Way.
VIRGOHI21
Main article: VIRGOHI21Initially discovered in 2000, VIRGOHI21 was announced in February 2005 as a good candidate to be a true dark galaxy. It was detected in 21-cm surveys, and was suspected to be a possible cosmic partner to the galaxy NGC 4254. This unusual-looking galaxy appears to be one partner in a cosmic collision, and appeared to show dynamics consistent with a dark galaxy (and apparently inconsistent with the predictions of the Modified Newtonian Dynamics (MOND) theory). However, further observations revealed that VIRGOHI21 was an intergalactic gas cloud, stripped from NGC4254 by a high speed collision. The high speed interaction was caused by infall into the Virgo cluster.
See also
- Dark matter – Concept in cosmology
- Dark matter halo – Theoretical cosmological structure
- Dwarf spheroidal galaxy – Low-luminosity galaxy of old stars & little dust
- Low-surface-brightness galaxy – Galaxy which is less bright than the ambient night skyPages displaying short descriptions of redirect targets
- Ultra diffuse galaxy – Extremely low luminosity galaxy
- Ultra faint dwarf galaxies – the most dark matter-dominated systems known
- Galaxy X (galaxy) – Dark satellite dwarf galaxy of the Milky Way Galaxy
References
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