VIRGOHI21

Infobox Galaxy
name =VIRGOHI 21


epoch = J2000
type =
ra = RA|12|17|53.6cite web
title=NASA/IPAC Extragalactic Database
work=Results for VIRGOHI 21
url=http://nedwww.ipac.caltech.edu/
accessdate=2006-11-19 ]
dec = DEC|+14|45|25
constellation name =Virgo
dist_ly = 50 Mlycite web |url=http://www.pparc.ac.uk/Nw/d_galaxy2.asp |title=New evidence for a Dark Matter Galaxy |accessdate=2008-02-26 |format= |publisher= Particle Physics and Astronomy Research Council (PPARC)]
z = 1966 km/s
appmag_v =
size_v =
notes = No stars
names =

VIRGOHI21 is a dark matter halo in the Virgo cluster discovered in 2005. It is the size of a galaxy, but apparently contains no stars.

Dark galaxy

"VIRGOHI21" was detected through H-I emissions of neutral hydrogen (21 cm emissions). This is the first discovery of the postulated dark matter clumps and galaxies anticipated by dark-matter theories. [cite journal | author=Minchin, Robert; Davies, Jonathan; Disney, Michael; Boyce, Peter; Garcia, Diego; Jordan, Christine; Kilborn, Virginia; Lang, Robert; Roberts, Sarah; Sabatini, Sabina; van Driel, Wim | title=A Dark Hydrogen Cloud in the Virgo Cluster | journal=Astrophysical Journal | year=2005 | volume=622 | issue=1 | pages=L21–L24 | doi=10.1086/429538]

The galaxy has a mass of 100 million to 10 billion solar masses, and is 50 million light-years away. The hydrogen gas present can only account for about one one-hundredth of the mass detected, which has been estimated by taking into account the rate of rotation of the dark matter halo. This is much more higher dark matter ratio then the standard ratio of one tenth ordinary matter found in normal galaxies.

Though other such dark matter galaxies were thought to have been found in the past, this is the first one to conclusively have almost no stars in it. A high resolution survey using the Hubble space telescope found only 119 red giant stars in the entire region of hydrogen gas.cite web |url=http://space.newscientist.com/article/dn12100-dark-galaxy-continues-to-puzzle-astronomers.html |title=Dark galaxy' continues to puzzle astronomers |accessdate=2008-02-26 |format= |publisher=New Scientist|author=Stuart Clark] The relative abundance of these dark-matter halos is still unknown, with someWho|date=July 2008 speculating that there could be more dark matter "galaxies" than regular ones.

Dark matter accounts for the majority of the universe's mass, and is present in "normal" galaxies as well. Indeed, the rotation and gluing together of normal galaxies is what led to the postulation of such dark matter. However, this galaxy appears to have a substantially higher percentage of dark matter than any galaxy discovered before. It is currently thought by most cosmologists that all galaxies came to be by clumps of dark matter condensing after the big bang, and consequently attracting hydrogen by the dark matter's gravitational attraction. Dark matter halos are different from our galaxy in that they failed to attract enough hydrogen to begin star birth.

The galaxy's strong gravity is thought to explain the drawn out arm of spiral galaxy NGC 4254, which was previously a mystery. Gas is being ripped from NGC 4254 and forms a bridge between the two entities. It is assumed that when the two galaxy sized objects have passed, that the extended arm of NGC 4254 will revert to normal.Who|date=March 2008

Alternate explanation

Scientists were skeptical of this finding because many dark galaxy candidates had been suggested before, but were explained as otherwise when higher resolution imagery was taken. Some scientists argued that the galaxy may be little more than tidal debris originating from the galaxy NGC 4254. [cite journal | author=Bekki, Kenji; Koribalski, Bärbel S.; Kilborn, Virginia A. | title=Dark Galaxies or Tidal Debris? Kinematical Clues to the Origin of Massive Isolated HI Clouds | journal=Monthly Notices of the Royal Astronomical Society | year=2005 | volume=363 | issue=1 | pages=L21–L25 | doi=10.1111/j.1745-3933.2005.00076.x]

A new, much more sensitive Arecibo map covering a much wider area, obtained by the [http://egg.astro.cornell.edu/alfalfa Arecibo Legacy Fast ALFA Survey] (ALFALFA), reveals that the HI cloud reported previously as VIRGOHI21 is only one condensation within a much more extensive tail clearly originating in NGC 4254. [cite journal | author=Haynes, Martha P.; Giovanelli, Riccardo; Kent, Brian R. | title= NGC 4254: An Act of Harassment Uncovered by the Arecibo Legacy Fast ALFA Survey | journal=Astrophysical Journal | year=2007 | volume=665 | issue=1 | pages=L19–22 | doi=10.1086/521188] Both the distribution of the HI gas and its velocity field can be reproduced in detail by a model involving NGC 4254 in a high speed collision with another galaxy which is now several degrees away. [cite journal | author=Duc, Pierre-Alain; Bournaud, Frederic | title= Tidal debris from high-velocity collisions as fake dark galaxies: A numerical model of VirgoHI21 | journal=Astrophysical Journal | year=2008 | volume= in press | issue=1 | pages= in press | doi=2007arXiv0710.3867D | doi_brokendate= 2008-06-22] Furthermore, other debris tails of this magnitude have been found to be common features in the Virgo cluster. [cite journal | author=Mihos, J. Christopher; Harding, Paul; Feldmeier, John; Morrison, Heather | title= Diffuse Light in the Virgo Cluster | journal=Astrophysical Journal | year=2005 | volume=631 | issue=1 | pages=L41–44 | doi=10.1086/497030] [cite journal | author=Oosterloo, T.; van Gorkom, J.| title= A large H I cloud near the centre of the Virgo cluster | journal=Astronomy and Astrophysics | year=2005 | volume=437 | issue=1 | pages=L19-22 | doi=10.1051/0004-6361:200500127},cite journal | author= Kent, Brian R.; Giovanelli, Riccardo; Haynes, Martha P.; Saintonge, Amelie; Stierwalt, Sabrina; Balonek, Thomas; Brosch, Noah; Catinella, Barbara; Koopmann, Rebecca A.; Momjian, E.; Spekkens, K.| title= Optically Unseen H i Detections toward the Virgo Cluster Detected in the Arecibo Legacy Fast ALFA Survey | journal=Astrophysical Journal | year=2007 | volume=665 | issue=1 | pages=L15-18| doi=10.1086/521100] These results suggest that VIRGOHI21 isn't such an unusual object after all, given its location at the edge of the densest region of the Virgo cluster.

This hypothesis is quite contentious however, and it remains difficult for astrophysicists to explain how the extended arm of NGC 4254 and large trail of debris could have formed without the presence of dark matter. According to Robert Minchin of the Arecibo Observatory, "If the hydrogen in VIRGOHI21 had been pulled out of a nearby galaxy, the same interaction should have pulled out stars as well".

ee also

* LSB galaxy
* Dark galaxy (astronomy)
* HVC 127-41-330

References

External links

* [http://news.bbc.co.uk/1/hi/wales/south_east/4288633.stm Astronomers find star-less galaxy] (BBC News), 2005
* [http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2004MNRAS.349..922D&db_key=AST&high=3f5efb5b2204761 A multibeam HI survey of the Virgo cluster - two isolated HI clouds?] , (abstract), Davies, J, et.al, 2004
* [http://www.astro.cf.ac.uk/groups/galaxies/dark.pdf A Dark Hydrogen Cloud in the Virgo Cluster]
* [http://space.com/scienceastronomy/050223_dark_galaxy.html First Invisible Galaxy Discovered in Cosmology Breakthrough] , (SPACE.com), 2005
* [http://www.theregister.co.uk/2005/02/23/galaxy_dark_first/ Astronomers spot first ever dark galaxy] (The Register), 2005
* [http://www.universetoday.com/am/publish/pparc_darkmatter_virgohi.html?1212006 Dark Matter Galaxy?] (UniverseToday)
* [http://www.spacedaily.com/reports/Arecibo_Survey_Produces_Dark_Galaxy_Candidate.html Arecibo Survey Produces Dark Galaxy Candidate] (SpaceDaily), 2006
* [http://www2.naic.edu/~rminchin/virgohi21.html 3D Animation from neutral Hydrogen data]