Home      Discussion      Topics      Dictionary      Almanac
Signup       Login
VIRGOHI21

VIRGOHI21

Overview
VIRGOHI21 is an extended region of neutral hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H2...

 (HI) in the Virgo cluster
Virgo Cluster
The Virgo Cluster is a cluster of galaxies at a distance of approximately 59 ± 4 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 hypothetical matter that is undetectable by its emitted radiation, but whose presence can be inferred from gravitational effects on visible matter...

, 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 name is from the Greek root galaxias [γαλαξίας], meaning "milky," a reference...

. Since VIRGOHI21 apparently contains no stars, this would make it one of the first detected dark galaxies
Dark galaxy
A dark galaxy is a galaxy-sized object containing very few or no stars . Held together by dark matter, it may also contain and dust. No dark galaxy with a black hole as a center has yet been discovered.-HE0450-2958:...

. Skeptics of this interpretation argue that VIRGOHI21 is simply a tidal tail of the nearby galaxy NGC 4254.

VIRGOHI21 was detected through radio telescope
Radio 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 spectral line created by changing in the energy state of neutral hydrogen and occurs at a frequency of 1420.40575177 MHz, equivalent to a vacuum wavelength of around 21.10611405413 cm...

.
Discussion
Ask a question about 'VIRGOHI21'
Start a new discussion about 'VIRGOHI21'
Answer questions from other users
Full Discussion Forum
 
Encyclopedia
VIRGOHI21 is an extended region of neutral hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H2...

 (HI) in the Virgo cluster
Virgo Cluster
The Virgo Cluster is a cluster of galaxies at a distance of approximately 59 ± 4 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 hypothetical matter that is undetectable by its emitted radiation, but whose presence can be inferred from gravitational effects on visible matter...

, 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 name is from the Greek root galaxias [γαλαξίας], meaning "milky," a reference...

. Since VIRGOHI21 apparently contains no stars, this would make it one of the first detected dark galaxies
Dark galaxy
A dark galaxy is a galaxy-sized object containing very few or no stars . Held together by dark matter, it may also contain and dust. No dark galaxy with a black hole as a center has yet been discovered.-HE0450-2958:...

. 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 telescope
Radio 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 spectral line created by changing in the energy state of neutral hydrogen and occurs at a frequency of 1420.40575177 MHz, equivalent to a vacuum wavelength of around 21.10611405413 cm...

. The detected hydrogen has a mass of about 100 million solar mass
Solar mass
The solar mass , , is a standard way to express mass in astronomy, used to describe the masses of other stars and galaxies. It is equal to the mass of the Sun, about two nonillion kilograms or about 332,950 times the mass of the Earth or 1,048 times the mass of Jupiter.The solar mass can be...

es and is about 50 million light-year
Light-year
As defined by the International Astronomical Union , a light-year is the distance that light travels in a vacuum in one Julian year.The speed of light is 299,792,458 metres per second...

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 the space shuttle in April 1990. It is named after the American astronomer Edwin Hubble. Although not the first space telescope, the Hubble is one of the largest and most versatile, and is well-known as both a vital...

 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 rotation
Galaxy 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 hypothetical matter that is undetectable by its emitted radiation, but whose presence can be inferred from gravitational effects on visible 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
Spiral galaxy
A spiral galaxy is a galaxy belonging to one of the three main classes 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...

 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 galaxy-sized object containing very few or no stars . Held together by dark matter, it may also contain and dust. No dark galaxy with a black hole as a center has yet been discovered.-HE0450-2958:...

 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 Telescope
Westerbork Synthesis Radio Telescope
The Westerbork Synthesis Radio Telescope is an aperture synthesis interferometer that consists of a linear array of 14 antennas arranged on a 2.7 km East-West line. It has a similar arrangement to other radio telescopes such as the One-Mile Telescope and the Ryle Telescope...

 (WSRT) and at Arecibo Observatory
Arecibo Observatory
The Arecibo Observatory is a radio telescope located close to the city of Arecibo in Puerto Rico. It is operated by Cornell University under cooperative agreement with the National Science Foundation. The observatory works as the National Astronomy and Ionosphere Center although both names are...

 revealed 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