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,[1] 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.[2] 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.[3] The best candidate dark galaxies to date include HI1225+01,[4] AGC229385,[5] 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, is made almost entirely of dark matter.[6][7][8]

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, i.e. 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 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 quasar.
Nature of dark galaxy
Origin

In 2005, astronomers found gas cloud VIRGOHI21 and attempted to determine what it was and why it caused such a gravitational pull on galaxy NGC 4254. After years of running out of other explanations, some have concluded that VIRGOHI21 is a dark galaxy, due to the massive effect it had on NGC 4254.[9]
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[10] to the size of a small quasar.
Structure

Dark galaxies are composed of dark matter. Furthermore, dark galaxies are theoretically composed of hydrogen and dust.[9] Some scientists support the idea that dark galaxies may contain stars.[11] Yet the exact composition of dark galaxies is unknown because there is no conclusive way to spot them so far. However, astronomers estimate that the mass of the gas in these galaxies is approximately 1 billion times that of the Sun.[12]
Methodology to observe dark bodies

Dark galaxies contain no visible stars, and are not visible using optical telescopes. The Arecibo Galaxy Environment Survey (AGES) is a current study using the Arecibo radio telescope to search for dark galaxies, which are predicted to contain detectable amounts of neutral hydrogen. The Arecibo radio telescope is useful where others are not because of its ability to detect the emission from this neutral hydrogen, specifically the 21 cm line.[13]
Alternative theories

Scientists do not have much explanation for some astronomic events, so some use the idea of a dark galaxy to explain these events. Little is known about dark galaxies, and some scientists believe a dark galaxy is actually a newly forming galaxy. One such candidate is in the Virgo cluster. This candidate contains very few stars. Scientists classify this galaxy as a newly forming galaxy, rather than a dark galaxy.[14] 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.”[12]
Potential dark galaxies
HE0450-2958
Main article: HE0450-2958

HE0450-2958 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.[15] 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,[16][17] so the observations do not support the dark galaxy interpretation.
HVC 127-41-330
Main article: HVC 127-41-330

HVC 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.[18][19]
Smith's Cloud
Main article: Smith's Cloud

Smith's Cloud is a candidate to be a dark galaxy, due to its projected mass and survival of encounters with the Milky Way.[20]
VIRGOHI21
Main article: VIRGOHI21

Initially discovered in 2000, VIRGOHI21 was announced in February 2005 as a good candidate to be a true dark galaxy.[11][21][22][23] 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).[24] However, further observations revealed that VIRGOHI21 was an intergalactic gas cloud, stripped from NGC4254 by a high speed collision.[25][26][27] The high speed interaction was caused by infall into the Virgo cluster.
See also

Dark matter – Hypothetical form of matter comprising most of the matter in the universe
Dark matter halo – A theoretical component of a galaxy that envelops the galactic disc and extends well beyond the edge of the visible galaxy
Dwarf spheroidal galaxy – Small, low-luminosity galaxies with very little dust and an older stellar population
Low-surface-brightness galaxy
Ultra diffuse galaxy – An extremely low luminosity galaxy
Galaxy X (galaxy)

References

"First evidence of dark galaxies from the early Universe spotted". Zmescience.com. 2012-07-11. Retrieved 2012-08-13.
Cannon, John M.; Martinkus, Charlotte P.; Leisman, Lukas; Haynes, Martha P.; Adams, Elizabeth A. K.; Giovanelli, Riccardo; Hallenbeck, Gregory; Janowiecki, Steven; Jones, Michael (2015-02-01). "The Alfalfa "Almost Darks" Campaign: Pilot VLA HI Observations of Five High Mass-To-Light Ratio Systems". The Astronomical Journal. 149 (2): 72.arXiv:1412.3018. Bibcode:2015AJ....149...72C. doi:10.1088/0004-6256/149/2/72. ISSN 0004-6256. S2CID 118592858.
Oosterloo, T. A.; Heald, G. H.; De Blok, W. J. G. (2013). "Is GBT 1355+5439 a dark galaxy?". Astronomy & Astrophysics. 555: L7.arXiv:1306.6148. Bibcode:2013A&A...555L...7O. doi:10.1051/0004-6361/201321965. S2CID 118402005.
"The Arecibo Legacy Fast ALFA Survey". egg.astro.cornell.edu. Retrieved 2015-09-29.
Janowiecki, Steven; Leisman, Lukas; Józsa, Gyula; Salzer, John J.; Haynes, Martha P.; Giovanelli, Riccardo; Rhode, Katherine L.; Cannon, John M.; Adams, Elizabeth A. K. (2015-03-01). "(Almost) Dark HI Sources in the ALFALFA Survey: The Intriguing Case of HI1232+20". The Astrophysical Journal. 801 (2): 96.arXiv:1502.01296. Bibcode:2015ApJ...801...96J. doi:10.1088/0004-637X/801/2/96. ISSN 0004-637X. S2CID 119271121.
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.
Hall, Shannon (25 August 2016). "Ghost galaxy is 99.99 per cent dark matter with almost no stars". New Scientist. Retrieved 27 August 2016.
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.
Fraser Cain (2007-06-14). "No Stars Shine in This Dark Galaxy". Universetoday.com. Retrieved 2012-08-13.
"Arecibo Survey Produces Dark Galaxy Candidate". Spacedaily.com. 2006-04-07. Retrieved 2012-08-13.
Stuart Clark. "Dark galaxy' continues to puzzle astronomers". New Scientist. Retrieved 2008-02-26.
"First Direct Detection Sheds Light On Dark Galaxies". Zmescience.com. Retrieved 22 December 2012.
in Astronomy (2009-12-23). "Invisible Dark-Matter Galaxy has Ten Billion Xs the Mass of the Sun". Dailygalaxy.com. Retrieved 2012-08-13.
"Drexler's Dark Matter Prediction Confirmation Followed His New Book". Newsblaze.com. 2009-11-30. Retrieved 2012-08-13.
Magain P.; et al. (2005). "Discovery of a bright quasar without a massive host galaxy". Nature. 437 (7057): 381–4.arXiv:astro-ph/0509433. Bibcode:2005Natur.437..381M. doi:10.1038/nature04013. PMID 16163349. S2CID 4303895.
Merritt, David; et al. (2006). "The nature of the HE0450-2958 system". Monthly Notices of the Royal Astronomical Society. 367 (4): 1746–1750.arXiv:astro-ph/0511315. Bibcode:2006MNRAS.367.1746M. doi:10.1111/j.1365-2966.2006.10093.x. S2CID 55834970.
Kim, Minjin; Ho, Luis; Peng, Chien; Im, Myungshin (2007). "The Host Galaxy of the Quasar HE 0450-2958". The Astrophysical Journal. 658 (1): 107–113.arXiv:astro-ph/0611411. Bibcode:2007ApJ...658..107K. doi:10.1086/510846. S2CID 14375599.
Josh Simon (2005). "Dark Matter in Dwarf Galaxies: Observational Tests of the Cold Dark Matter Paradigm on Small Scales" (PDF): 4273. Bibcode:2005PhDT.........2S. Archived from the original (PDF) on 2006-09-13.
Battersby, Stephen (2003-10-20). "Astronomers find first 'dark galaxy'". New Scientist. Retrieved December 22, 2012.
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Clark, Stuart (2005-02-23). "Astronomers claim first 'dark galaxy' find". NewScientist.com news service. Retrieved 2006-10-26.
Shiga, David (2005-02-26). "Ghostly Galaxy: Massive, dark cloud intrigues scientists". Science News Online. 167 (9): 131. doi:10.2307/4015891. JSTOR 4015891. Archived from the original on 2011-05-24. Retrieved 2008-09-14.
Britt, Roy (2005-02-23). "First Invisible Galaxy Discovered in Cosmology Breakthrough". Space.com.
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Kent, Brian R.; Giovanelli, Riccardo; Haynes, Martha P.; Saintonge, Amélie; Stierwalt, Sabrina; Balonek, Thomas; Brosch, Noah; Catinella, Barbara; Koopmann, Rebecca A. (2007-08-01). "Optically Unseen H I Detections toward the Virgo Cluster Detected in the Arecibo Legacy Fast ALFA Survey". The Astrophysical Journal Letters. 665 (1): L15–L18. Bibcode:2007ApJ...665L..15K. doi:10.1086/521100. ISSN 0004-637X.
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External links

Universe Today, Some Galaxies Are Made Almost Entirely of Dark Matter

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Galaxies
Morphology

Disc Lenticular
barred unbarred Spiral
anemic barred flocculent grand design intermediate Magellanic unbarred Dwarf galaxy
elliptical irregular spheroidal spiral Elliptical galaxy
cD Irregular
barred Peculiar Ring
Polar

Structure

Active galactic nucleus Bar Bulge Dark matter halo Disc
Disc galaxy Halo
corona Galactic center Galactic plane Galactic ridge Interstellar medium Protogalaxy Spiral arm Supermassive black hole

Active nuclei

Blazar LINER Markarian Quasar Radio
X-shaped Relativistic jet Seyfert

Energetic galaxies

Lyman-alpha emitter Luminous infrared Starburst
blue compact dwarf pea faint blue Hot dust-obscured

Low activity

Low surface brightness Ultra diffuse Dark galaxy

Interaction

Field Galactic tide Cloud Groups and clusters
group cluster Brightest cluster galaxy fossil group Interacting
merger Jellyfish Satellite Stellar stream Superclusters Walls Voids and supervoids
void galaxy

Lists

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Superclusters Voids