Magellanic Stream
Encyclopedia
The Magellanic Stream is a high-velocity cloud of gas connecting the Large
and the Small
Magellanic Clouds
. It came into existence by a near-collision of both galaxies
some 2.5 billion years ago.
were found in the region of the Magellanic Clouds. The gas stretches for at least 180 degrees across the sky. This corresponds to 180 kpc
(600,000 ly) at an approximate distance of 55 kpc
(180,000 ly). The gas is very collimated and polar with respect to the Milky Way
. The velocity range is huge (from -400 to 400 km s−1 in reference to Local Standard of Rest
). Velocity patterns do not follow the rest of the Milky Way. So it was determined to be a classic High velocity cloud.
But the gas was not mapped, and the connection to the two Magellanic Clouds was not made. The Magellanic Stream as such was discovered as a Neutral Hydrogen (HI)
gas feature near the Magellanic Clouds
by Wannier & Wrixon in 1972. Its connection to the Magellanic Clouds was made by Mathewson et al. in 1974.
Owing to the closeness of the Magellanic Clouds and the ability to resolve individual stars and their parallax
es, subsequent observations gave the full 6 dimensional phase space
information of both clouds (with very large relative errors for the transverse velocities). This enabled the calculation of the likely past orbit of the Large and the Small Magellanic Cloud in relation to the Milky Way. The calculation necessitated large assumptions, for example, on the shapes and masses of the 3 galaxies, and the nature of dynamical friction
between the moving objects. Observations of individual stars revealed details of star formation history.
s describing the formation of the Magellanic Stream had been produced since 1980. Following computing power, the initial models were very simple, non-self gravitating, and with few particle
s. Most models predicted a feature leading the Magellanic Clouds. These early models were 'tidal' models. Just like tide
s on Earth are induced by the gravity of the 'leading' Moon
, the models predicted two directions opposite each other, in which particles are preferentially pulled. However, the predicted features were not observed. This led to a few models that did not require a leading element but which had problems of their own. In 1998 a study analysing the full sky survey made by the HIPASS
team at Parkes Observatory
generated important new observational data. Putman et al. discovered that a mass of High Velocity Clouds leading the Magellanic Clouds was actually fully connected to the Magellanic Clouds. So, the Leading Arm Feature had its existence finally established. Furthermore, Lu et al. (1998( and Gibson et al. (2000) established the chemical similarity between the streams and Magellanic Clouds.
Newer, increasingly sophisticated models all tested the Leading Arm Feature hypothesis. These models make heavy use of gravity effects through tidal fields. Some models also rely on ram pressure
stripping as a shaping mechanism. Most recent models increasingly include drag from the halo
of the Milky Way Galaxy as well as gas dynamics
, star formation
and chemical evolution. It is thought that the tidal forces mostly affect the Small Magellanic Cloud, since it has lower mass, and is less gravitationally bound. In contrast, ram pressure stripping mostly affects the Large Magellanic Cloud, because it has a larger reservoir of gas.
, David Nidever of the University of Virginia
announced new results based on data derived from the National Science Foundation
’s Robert C. Byrd Green Bank Telescope
and earlier radio astronomy
observations. Now it appears established that the Magellanic Stream is much longer than earlier thought. Specifically, its size allowes an explanation that Magellanic Stream had its beginning at about the time when the two Magellanic Clouds may have passed close to each other some 2.5 billion years ago. This event triggered massive bursts of star formation
Large Magellanic Cloud
The Large Magellanic Cloud is a nearby irregular galaxy, and is a satellite of the Milky Way. At a distance of slightly less than 50 kiloparsecs , the LMC is the third closest galaxy to the Milky Way, with the Sagittarius Dwarf Spheroidal and Canis Major Dwarf Galaxy lying closer to the center...
and the Small
Small Magellanic Cloud
The Small Magellanic Cloud is a dwarf galaxy. It has a diameter of about 7,000 light-years and contains several hundred million stars. It has a total mass of approximately 7 billion times the mass of our Sun....
Magellanic Clouds
Magellanic Clouds
The two Magellanic Clouds are irregular dwarf galaxies visible in the southern hemisphere, which are members of our Local Group and are orbiting our Milky Way galaxy...
. It came into existence by a near-collision of both galaxies
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...
some 2.5 billion years ago.
Discovery and early observations
In 1965, anomalous velocity gas cloudsInterstellar cloud
Interstellar cloud is the generic name given to an accumulation of gas, plasma and dust in our and other galaxies. Put differently, an interstellar cloud is a denser-than-average region of the interstellar medium. Depending on the density, size and temperature of a given cloud, the hydrogen in it...
were found in the region of the Magellanic Clouds. The gas stretches for at least 180 degrees across the sky. This corresponds to 180 kpc
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....
(600,000 ly) at an approximate distance of 55 kpc
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....
(180,000 ly). The gas is very collimated and polar with respect to the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
. The velocity range is huge (from -400 to 400 km s−1 in reference to Local Standard of Rest
Local standard of rest
In astronomy, the local standard of rest or LSR follows the mean motion of material in the Milky Way in the neighborhood of the Sun. The path of this material is not precisely circular...
). Velocity patterns do not follow the rest of the Milky Way. So it was determined to be a classic High velocity cloud.
But the gas was not mapped, and the connection to the two Magellanic Clouds was not made. The Magellanic Stream as such was discovered as a Neutral Hydrogen (HI)
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...
gas feature near the Magellanic Clouds
Magellanic Clouds
The two Magellanic Clouds are irregular dwarf galaxies visible in the southern hemisphere, which are members of our Local Group and are orbiting our Milky Way galaxy...
by Wannier & Wrixon in 1972. Its connection to the Magellanic Clouds was made by Mathewson et al. in 1974.
Owing to the closeness of the Magellanic Clouds and the ability to resolve individual stars and their parallax
Parallax
Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek παράλλαξις , meaning "alteration"...
es, subsequent observations gave the full 6 dimensional phase space
Phase space
In mathematics and physics, a phase space, introduced by Willard Gibbs in 1901, is a space in which all possible states of a system are represented, with each possible state of the system corresponding to one unique point in the phase space...
information of both clouds (with very large relative errors for the transverse velocities). This enabled the calculation of the likely past orbit of the Large and the Small Magellanic Cloud in relation to the Milky Way. The calculation necessitated large assumptions, for example, on the shapes and masses of the 3 galaxies, and the nature of dynamical friction
Dynamical friction
Dynamical friction is a term in astrophysics related to loss of momentum and kinetic energy of moving bodies through a gravitational interaction with surrounding matter in space...
between the moving objects. Observations of individual stars revealed details of star formation history.
Models
ModelScientific modelling
Scientific modelling is the process of generating abstract, conceptual, graphical and/or mathematical models. Science offers a growing collection of methods, techniques and theory about all kinds of specialized scientific modelling...
s describing the formation of the Magellanic Stream had been produced since 1980. Following computing power, the initial models were very simple, non-self gravitating, and with few particle
Particle
A particle is, generally, a small localized object to which can be ascribed physical properties. It may also refer to:In chemistry:* Colloidal particle, part of a one-phase system of two or more components where the particles aren't individually visible.In physics:* Subatomic particle, which may be...
s. Most models predicted a feature leading the Magellanic Clouds. These early models were 'tidal' models. Just like tide
Tide
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the moon and the sun and the rotation of the Earth....
s on Earth are induced by the gravity of the 'leading' Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
, the models predicted two directions opposite each other, in which particles are preferentially pulled. However, the predicted features were not observed. This led to a few models that did not require a leading element but which had problems of their own. In 1998 a study analysing the full sky survey made by the HIPASS
HIPASS
The HI Parkes All Sky Survey was an astronomical survey for neutral atomic hydrogen . Data was taken between 1997 and 2002 using the Parkes Observatory. HIPASS covered 71% of the sky and identified 5317 sources emitting HI's signature wavelength...
team at Parkes Observatory
Parkes Observatory
The Parkes Observatory is a radio telescope observatory, 20 kilometres north of the town of Parkes, New South Wales, Australia. It was one of several radio antennas used to receive live, televised images of the Apollo 11 moon landing on 20 July 1969....
generated important new observational data. Putman et al. discovered that a mass of High Velocity Clouds leading the Magellanic Clouds was actually fully connected to the Magellanic Clouds. So, the Leading Arm Feature had its existence finally established. Furthermore, Lu et al. (1998( and Gibson et al. (2000) established the chemical similarity between the streams and Magellanic Clouds.
Newer, increasingly sophisticated models all tested the Leading Arm Feature hypothesis. These models make heavy use of gravity effects through tidal fields. Some models also rely on ram pressure
Ram pressure
In physics, ram pressure is a pressure exerted on a body which is moving through a fluid medium. It causes a strong drag force to be exerted on the body. It is given by:P= \rho v^2...
stripping as a shaping mechanism. Most recent models increasingly include drag from the halo
Galactic halo
The term galactic halo is used to denote an extended, roughly spherical component of a galaxy, which extends beyond the main, visible component. It can refer to any of several distinct components which share these properties:* the galactic spheroid...
of the Milky Way Galaxy as well as gas dynamics
Gas dynamics
Gas dynamics is a branch of fluid dynamics concerned with studying the motion of gases and its consequent effects. Gas dynamics combines the principles of fluid mechanics and thermodynamics...
, star formation
Star formation
Star formation is the process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. As a branch of astronomy star formation includes the study of the interstellar medium and giant molecular clouds as precursors to the star formation process and the study of young...
and chemical evolution. It is thought that the tidal forces mostly affect the Small Magellanic Cloud, since it has lower mass, and is less gravitationally bound. In contrast, ram pressure stripping mostly affects the Large Magellanic Cloud, because it has a larger reservoir of gas.
New results on its formation
At the January 2010 meeting of the American Astronomical SocietyAmerican Astronomical Society
The American Astronomical Society is an American society of professional astronomers and other interested individuals, headquartered in Washington, DC...
, David Nidever of the University of Virginia
University of Virginia
The University of Virginia is a public research university located in Charlottesville, Virginia, United States, founded by Thomas Jefferson...
announced new results based on data derived from the National Science Foundation
National Science Foundation
The National Science Foundation is a United States government agency that supports fundamental research and education in all the non-medical fields of science and engineering. Its medical counterpart is the National Institutes of Health...
’s Robert C. Byrd Green Bank Telescope
Green Bank Telescope
The Robert C. Byrd Green Bank Telescope is the world's largest fully steerable radio telescope and the world's largest land-based movable structure. It is part of the National Radio Astronomy Observatory site at Green Bank, West Virginia, USA. The telescope honors the name of the late Senator...
and earlier radio astronomy
Radio astronomy
Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The initial detection of radio waves from an astronomical object was made in the 1930s, when Karl Jansky observed radiation coming from the Milky Way. Subsequent observations have identified a number of...
observations. Now it appears established that the Magellanic Stream is much longer than earlier thought. Specifically, its size allowes an explanation that Magellanic Stream had its beginning at about the time when the two Magellanic Clouds may have passed close to each other some 2.5 billion years ago. This event triggered massive bursts of star formation
Star formation
Star formation is the process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. As a branch of astronomy star formation includes the study of the interstellar medium and giant molecular clouds as precursors to the star formation process and the study of young...
External links
- The Magellanic Stream, Astronomy Picture of the DayAstronomy Picture of the DayAstronomy Picture of the Day is a website provided by NASA and Michigan Technological University . According to the website, "Each day a different image or photograph of our universe is featured, along with a brief explanation written by a professional astronomer."The photograph is not necessarily...
2010 January 25