hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...
(HI) in the
Virgo cluster
The Virgo Cluster is a large cluster of galaxies whose center is 53.8 ± 0.3 Mly (16.5 ± 0.1 Mpc) away in the constellation Virgo. Comprising approximately 1,300 (and possibly up to 2,000) member galaxies, the cluster forms the heart of the lar ...
discovered in 2005. Analysis of its internal motion indicates that it may contain a large amount of
dark matter
Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not ab ...
, as much as a small
galaxy
A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ...
. Since VIRGOHI21 apparently contains no stars, this would make it one of the first detected
dark galaxies
A 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 ...
. Skeptics of this interpretation argue that VIRGOHI21 is simply a
tidal tail
A tidal tail is a thin, elongated region of stars and interstellar gas that extends into space from a galaxy. Tidal tails occur as a result of galactic tide forces between interacting galaxies. Examples of galaxies with tidal tails include the ...
radio telescope
A radio telescope is a specialized antenna and radio receiver used to detect radio waves from astronomical radio sources in the sky. Radio telescopes are the main observing instrument used in radio astronomy, which studies the radio frequency ...
observations of its neutral hydrogen 21 cm emissions. The detected hydrogen has a mass of about 100 million
solar mass
The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass ...
es and is about 50 million
light-year
A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...
s away. By analyzing the
Doppler shift
The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who d ...
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
The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versa ...
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
Rotation, or spin, is the circular movement of an object around a '' central axis''. A two-dimensional rotating object has only one possible central axis and can rotate in either a clockwise or counterclockwise direction. A three-dimensional ...
, 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 is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not ab ...
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 galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae''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
A 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 ...
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
The Westerbork Synthesis Radio Telescope (WSRT) is an aperture synthesis interferometer built on the site of the former World War II Nazi detention and transit camp Westerbork, north of the village of Westerbork, Midden-Drenthe, in the northea ...
(WSRT) and at
Arecibo Observatory
The Arecibo Observatory, also known as the National Astronomy and Ionosphere Center (NAIC) and formerly known as the Arecibo Ionosphere Observatory, is an observatory in Barrio Esperanza, Arecibo, Puerto Rico owned by the US National Science F ...
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.
See also
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LSB galaxy
A low-surface-brightness galaxy, or LSB galaxy, is a diffuse galaxy with a surface brightness that, when viewed from Earth, is at least one magnitude lower than the ambient night sky.
Most LSBs are dwarf galaxies, and most of their baryonic matt ...
*
HVC 127-41-330
HVC 127-41-330 is a high-velocity cloud in the constellation of Pisces. The three numbers that compose its name indicate, respectively, the galactic longitude and latitude, and velocity towards Earth in km/s. It is 20,000 light years in diameter a ...