Dark galaxy
Encyclopedia
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. An influential community of scientists conjecture the existence of dark galaxies to support theories based on other well-studied celestial bodies.
There is experimental evidence to support the existence of dark galaxies, although scientists have no conclusive evidence and continue their research. Scientists have proposed a means to track down the dark dwarf galaxies that should be orbiting the Milky Way, saying they have now found evidence of one.
and tried to find a theory of what it was and or why it could cause such a gravitational pull from NGC 4254 galaxy. 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.
. Furthermore, dark galaxies are theoretically composed of hydrogen and dust
. Some scientists support the idea that dark galaxies may contain stars. Yet the exact composition of dark galaxies is unknown because there is no conclusive way to spot them so far.
, rather than a dark galaxy.
(a star like object that may send out radio waves and other forms of energy). This one in particular has many large red shifts. HE0450-2958 has no visible host galaxy (a galaxy surrounding the quasar) detected around it. It has been suggested that this may be a dark galaxy in which a quasar has become active. However subsequent observations revealed that a normal host galaxy is probably present.
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.
theory. Some researchers have since discounted the possibility of VIRGOHI21 being a dark galaxy and believe it is more likely a "tidal tail
" of nearby galaxy NGC 4254, which is experiencing gravitational perturbations as it enters the Virgo cluster
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...
composed 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...
. Dark galaxies receive their name because they have no detectable stars and are theoretically invisible. An influential community of scientists conjecture the existence of dark galaxies to support theories based on other well-studied celestial bodies.
There is experimental evidence to support the existence of dark galaxies, although scientists have no conclusive evidence and continue their research. Scientists have proposed a means to track down the dark dwarf galaxies that should be orbiting the Milky Way, saying they have now found evidence of one.
Observational evidence
Astronomers first suspected that there was an invisible galaxy, dark galaxy, out there when they spied galaxy NGC 4254. This unusual-looking galaxy appears to be one partner in a cosmic collision. The only evidence is the following: gas is being siphoned away into a tenuous stream, and one of its spiral arms is being stretched out. The other partner in this collision is nowhere to be seen. The researchers calculated that an object with 100 billion solar masses theoretically careened past NGC 4254 within the last 100 million years creating the gas stream and tearing at one of its arms. This was the clue that an invisible dark matter galaxy might be lurking nearby.Origin
In 2000 astronomers found a gas cloud VIRGOHI21VIRGOHI21
VIRGOHI21 is an extended region of neutral hydrogen 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...
and tried to find a theory of what it was and or why it could cause such a gravitational pull from NGC 4254 galaxy. 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.
Size
The actual size of a dark galaxy is unknown, because they cannot be spotted with a normal telescope. There have been various estimations of the size of dark galaxies. Two potential sizes could be either double the size of the Milky Way or the size of a small quasar.Structure
Dark galaxies are composed of dark matterDark 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...
. Furthermore, dark galaxies are theoretically composed of hydrogen and dust
Cosmic dust
Cosmic dust is a type of dust composed of particles in space which are a few molecules to 0.1 µm in size. Cosmic dust can be further distinguished by its astronomical location; for example: intergalactic dust, interstellar dust, interplanetary dust and circumplanetary dust .In our own Solar...
. Some scientists support the idea that dark galaxies may contain stars. Yet the exact composition of dark galaxies is unknown because there is no conclusive way to spot them so far.
Methodology to observe dark bodies
Dark galaxies contain no visible stars, and are not visible through conventional methods, i.e. telescopes. Arecibo Galaxy Environment Survey (AGES) a current study using the Arecibo radio telescope to search for dark galaxies. The Arecibo radio telescope is useful where others are not because of its ability to detect neutral-hydrogen wavelengths.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 dark galaxy is actually a newly forming galaxy. One such candidate is in the Virgo cluster. This candidate contains very few stars. Scientist classify this galaxy as a newly forming galaxyGalaxy formation and evolution
The study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universe from a homogeneous beginning, the formation of the first galaxies, the way galaxies change over time, and the processes that have generated the variety of structures observed in nearby...
, rather than a dark galaxy.
HE0450-2958
HE0450-2958 is an unusual quasarQuasar
A quasi-stellar radio source is a very energetic and distant active galactic nucleus. Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than...
(a star like object that may send out radio waves and other forms of energy). This one in particular has many large red shifts. HE0450-2958 has no visible host galaxy (a galaxy surrounding the quasar) detected around it. It has been suggested that this may be a dark galaxy in which a quasar has become active. However subsequent observations revealed that a normal host galaxy is probably present.
HVC 127-41-330
HVC 127-41-330 is a cloud at high speed between the AndromedaAndromeda Galaxy
The Andromeda Galaxy is a spiral galaxy approximately 2.5 million light-years from Earth in the constellation Andromeda. It is also known as Messier 31, M31, or NGC 224, and is often referred to as the Great Andromeda Nebula in older texts. Andromeda is the nearest spiral galaxy to the...
and the Triangulum Galaxy
Triangulum Galaxy
The Triangulum Galaxy is a spiral galaxy approximately 3 million light years from Earth in the constellation Triangulum. It is catalogued as Messier 33 or NGC 598, and is sometimes informally referred to as the Pinwheel Galaxy, a nickname it shares with Messier 101...
. Astronomer Josh Simon considers this cloud to be a dark galaxy because of the speed of its rotation and its predicted mass.
VIRGOHI21
The discovery of VIRGOHI21 was announced in February 2005, and it was the first good candidate to be a true dark galaxy. It was found when AGES was looking for the 21 cm-wavelength radio waves emitted by hydrogen (H). Its dynamics are apparently inconsistent with the predictions of the Modified Newtonian Dynamics (MOND)Modified Newtonian dynamics
In physics, Modified Newtonian dynamics is a hypothesis that proposes a modification of Newton's law of gravity to explain the galaxy rotation problem. When the uniform velocity of rotation of galaxies was first observed, it was unexpected because Newtonian theory of gravity predicts that objects...
theory. Some researchers have since discounted the possibility of VIRGOHI21 being a dark galaxy and believe it is more likely a "tidal tail
Galactic tide
A galactic tide is a tidal force experienced by objects subject to the gravitational field of a galaxy such as the Milky Way. Particular areas of interest concerning galactic tides include galactic collisions, the disruption of dwarf or satellite galaxies, and the Milky Way's tidal effect on the...
" of nearby galaxy NGC 4254, which is experiencing gravitational perturbations as it enters 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...
See also
- Low surface brightness galaxyLow surface brightness galaxyA 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....
- Dwarf spheroidal galaxyDwarf spheroidal galaxyDwarf spheroidal galaxy is a term in astronomy applied to low luminosity galaxies that are companions to the Milky Way and to the similar systems that are companions to the Andromeda Galaxy M31...
- Dark matter haloDark matter haloA dark matter halo is a hypothetical component of a galaxy, which extends beyond the edge of the visible galaxy and dominates the total mass. Since they consist of dark matter, halos cannot be observed directly, but their existence is inferred through their effects on the motions of stars and gas...
- Dark matterDark matterIn 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...
External links
- Universe Today, Some Galaxies Are Made Almost Entirely of Dark Matter