VIRGOHI21
Encyclopedia
VIRGOHI21 is an extended region of neutral hydrogen
(HI) in the Virgo cluster
discovered in 2005. Analysis of its internal motion indicates that it may contain a large amount of dark matter
, as much as a small galaxy
. Since VIRGOHI21 apparently contains no stars, this would make it one of the first detected dark galaxies
. Skeptics of this interpretation argue that VIRGOHI21 is simply a tidal tail of the nearby galaxy NGC 4254.
observations of its neutral hydrogen 21 cm emissions
. The detected hydrogen has a mass of about 100 million solar mass
es and is about 50 million light-year
s away. By analyzing the Doppler shift of the emissions, astronomers determined that the gas has a high velocity-profile width; that is, different parts of the cloud are moving at high speed relative to other parts. Follow-up Hubble space telescope
deep observations of the region have detected very few stars (a few hundred).
, it is far too fast to be consistent with the gravity of the detected hydrogen. Rather, it implies the presence of a dark matter
halo with tens of billions of solar masses. Given the very small number of stars detected, this implies a mass-to-light ratio of about 500, far greater than that of a normal galaxy (around 50). The large gravity of the dark matter halo in this interpretation explains the perturbed nature of the nearby spiral galaxy
NGC 4254 and the bridge of neutral hydrogen extending between the two entities.
Under this interpretation, VIRGOHI21 would be the first discovery of the dark galaxies
anticipated by simulations of dark-matter theories. Although other dark-galaxy candidates have previously been observed, follow-up observations indicated that these were either very faint ordinary galaxies or tidal tails. VIRGOHI21 is considered the best current candidate for a dark galaxy.
(WSRT) and at Arecibo Observatory
revealed that VIRGOHI21 is embedded within a much more extensive tail originating in NGC 4254. Both the distribution of the HI gas and its velocity field can be reproduced by a model involving NGC 4254 in a high-speed collision with another galaxy (probably NGC 4192), which is now somewhat distant. Other debris tails of this magnitude have been found to be common features in the Virgo cluster, where the high density of galaxies makes interactions frequent. These results suggest that VIRGOHI21 is not an unusual object, given its location at the edge of the densest region of the Virgo cluster.
The original paper describing VIRGOHI21 as a dark galaxy provides several objections to the tidal-tail interpretation: that high-velocity interactions do not generally produce significant tails, that the high velocity needed is out-of-place in this part of the Virgo cluster and that the observed velocity profile is opposite from that expected in a tidal tail. In addition, 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". Proponents of the tidal-tail interpretation counter these objections with simulations and argue that the apparently inverted velocity profile is due to the orientation of the tail with respect to Earth-based observers.
Although the nature of VIRGOHI21 remains a contentious issue, its identification as a dark galaxy seems much less certain now than immediately after its discovery.
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
(HI) in the Virgo cluster
Virgo Cluster
The Virgo Cluster is a cluster of galaxies whose center is 53.8 ± 0.3 Mly away in the constellation Virgo. Comprising approximately 1300 member galaxies, the cluster forms the heart of the larger Local Supercluster, of which the Local Group is an outlying member...
discovered in 2005. Analysis of its internal motion indicates that it may contain a large amount of dark matter
Dark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...
, as much as a small galaxy
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
. Since VIRGOHI21 apparently contains no stars, this would make it one of the first detected dark galaxies
Dark galaxy
A dark galaxy is a hypothetical galaxy composed of dark matter. Dark galaxies receive their name because they have no detectable stars and are theoretically invisible...
. Skeptics of this interpretation argue that VIRGOHI21 is simply a tidal tail of the nearby galaxy NGC 4254.
Observational properties
VIRGOHI21 was detected through radio telescopeRadio telescope
A radio telescope is a form of directional radio antenna used in radio astronomy. The same types of antennas are also used in tracking and collecting data from satellites and space probes...
observations of its neutral hydrogen 21 cm emissions
Hydrogen line
The hydrogen line, 21 centimeter line or HI line refers to the electromagnetic radiation spectral line that is created by a change in the energy state of neutral hydrogen atoms. This electromagnetic radiation is at the precise frequency of 1420.40575177 MHz, which is equivalent to the vacuum...
. The detected hydrogen has a mass of about 100 million solar mass
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
es and is about 50 million light-year
Light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...
s away. By analyzing the Doppler shift of the emissions, astronomers determined that the gas has a high velocity-profile width; that is, different parts of the cloud are moving at high speed relative to other parts. Follow-up Hubble space telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
deep observations of the region have detected very few stars (a few hundred).
Dark galaxy interpretation
If the high velocity-profile width of VIRGOHI21 is interpreted as rotationGalaxy rotation curve
The rotation curve of a galaxy can be represented by a graph that plots the orbital velocity of the stars or gas in the galaxy on the y-axis against the distance from the center of the galaxy on the x-axis....
, it is far too fast to be consistent with the gravity of the detected hydrogen. Rather, it implies the presence of a dark matter
Dark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...
halo with tens of billions of solar masses. Given the very small number of stars detected, this implies a mass-to-light ratio of about 500, far greater than that of a normal galaxy (around 50). The large gravity of the dark matter halo in this interpretation explains the perturbed nature of the nearby spiral galaxy
Spiral galaxy
A spiral galaxy is a certain kind of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as...
NGC 4254 and the bridge of neutral hydrogen extending between the two entities.
Under this interpretation, VIRGOHI21 would be the first discovery of the dark galaxies
Dark galaxy
A dark galaxy is a hypothetical galaxy composed of dark matter. Dark galaxies receive their name because they have no detectable stars and are theoretically invisible...
anticipated by simulations of dark-matter theories. Although other dark-galaxy candidates have previously been observed, follow-up observations indicated that these were either very faint ordinary galaxies or tidal tails. VIRGOHI21 is considered the best current candidate for a dark galaxy.
Tidal tail interpretation
Sensitive maps covering a much wider area, obtained at Westerbork Synthesis Radio TelescopeWesterbork Synthesis Radio Telescope
The Westerbork Synthesis Radio Telescope is an aperture synthesis interferometer near camp Westerbork, north of the village of Westerbork, Midden-Drenthe, in the northeastern Netherlands. It consists of a linear array of 14 antennas with a diameter of 25 metres arranged on a 2.7 km East-West line...
(WSRT) and at Arecibo Observatory
Arecibo Observatory
The Arecibo Observatory is a radio telescope near the city of Arecibo in Puerto Rico. It is operated by SRI International under cooperative agreement with the National Science Foundation...
revealed that VIRGOHI21 is embedded within a much more extensive tail originating in NGC 4254. Both the distribution of the HI gas and its velocity field can be reproduced by a model involving NGC 4254 in a high-speed collision with another galaxy (probably NGC 4192), which is now somewhat distant. Other debris tails of this magnitude have been found to be common features in the Virgo cluster, where the high density of galaxies makes interactions frequent. These results suggest that VIRGOHI21 is not an unusual object, given its location at the edge of the densest region of the Virgo cluster.
The original paper describing VIRGOHI21 as a dark galaxy provides several objections to the tidal-tail interpretation: that high-velocity interactions do not generally produce significant tails, that the high velocity needed is out-of-place in this part of the Virgo cluster and that the observed velocity profile is opposite from that expected in a tidal tail. In addition, 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". Proponents of the tidal-tail interpretation counter these objections with simulations and argue that the apparently inverted velocity profile is due to the orientation of the tail with respect to Earth-based observers.
Although the nature of VIRGOHI21 remains a contentious issue, its identification as a dark galaxy seems much less certain now than immediately after its discovery.
External links
- Astronomers find star-less galaxy (BBC News), 2005
- A multibeam HI survey of the Virgo cluster - two isolated HI clouds?, (abstract), Davies, J, et al., 2004
- A Dark Hydrogen Cloud in the Virgo Cluster
- First Invisible Galaxy Discovered in Cosmology Breakthrough, (SPACE.com), 2005
- Astronomers spot first ever dark galaxy (The Register), 2005
- Dark Matter Galaxy? (UniverseToday)
- Arecibo Survey Produces Dark Galaxy Candidate (SpaceDaily), 2006
- 3D Animation from neutral Hydrogen data