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Encyclopedia > Magellanic Stream

Contents


Introduction

Hierarchical Clustering tells us that galaxies are built up over time from collisions of smaller galaxies. These collisions are still going on today, with the Milky Way still cannibalising its smaller neighbours. The best known and most studied example of these mergers, is the Magellanic Stream. The Magellanic Stream (MS) was discovered as a Neutral Hydrogen (HI) gas feature near the Magellanic Clouds (MCs) by Wannier & Wrixon in 1972, and the connection to the MCs was made by Mathewson et al. in 1974. Previous to that, in 1965, [[High velocity cloud|anomalous velocity]] gas clouds were known in that region, but the gas was not mapped, and the connection to the MCs was not made. The gas was seen to be a very long (at least 180 degrees across the sky - corresponding to 180 kpc (600,000 lyr) long at an approximate distance of 55 kpc (180,000 lyr), and very collimated, and polar (with respect to the Galaxy) in nature. The velocity range was huge from -400 to 400kms − 1 in the [[Local Standard of Rest]], and wasn't following the velocity patterns of the rest of the galaxy -- it was a classic High velocity cloud. The hydrogen line refers to the spectral line created by changes in the energy state of neutral hydrogen and occurs at 1420. ... The two Magellanic Clouds are irregular dwarf galaxies orbiting our Milky Way galaxy, and thus are members of our Local Group of galaxies. ... Interstellar cloud is the generic name given to accumulations of gas and dust in our galaxy. ... The parsec (symbol pc) is a unit of length used in astronomy. ... A light year, abbreviated ly, is the distance light travels in one year: roughly 9. ... The parsec (symbol pc) is a unit of length used in astronomy. ... A light year, abbreviated ly, is the distance light travels in one year: roughly 9. ...


Observations

Owing to the closeness of the MCs, and the ability to resolve individual stars and their parallaxes, observations gave us the full 6 dimensional phase space information of both clouds (with very large relative errors for the transverse velocities), which enabled the calculation of the likely past orbit of them (with large assumptions such as the shapes and masses of the 3 galaxies, and the nature of dynamical friction between the moving objects). Observations of individual stars gave us the star formation history. Parallax (Greek: παραλλαγή (parallagé) = alteration) is the change of angular position of two stationary points relative to each other as seen by an observer, due to the motion of said observer. ... Phase space is a useful construct in mathematics and physics to demonstrate and visualise the changes in the dynamical variables of a system. ... 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. ...


Models

Models had been produced for the formation of the MS since 1980. Initially, following computing power, the models were very simple, non-self gravitating, and with few particles. Most models predicted a feature leading the MCs (they were tidal models, and just like tides on the earth, the models predict two directions opposite each other, in which material is preferentially pulled), but this was not observed, leading to a few models which didn't require the leading arm, but which had problems of their own. In 1998, subsequent to a full sky survey made by the HIPASS team at the Parkes Radio telescope, Putman et al. discovered that a mass of High Velocity Clouds leading the MCs were actually fully connected to the MCs, and so the Leading Arm Feature (LAF) 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.


New models since then have all had to make sure this LAF existed, and the models have been getting increasingly sophisticated. Most of them make heavy use of gravity, through tidal fields (although some rely on ram pressure stripping for the shaping mechanism), and more and more are increasingly including drag from the Milky Way Galaxy halo, gas dynamics, star formation and chemical evolution. It is thought that the tidal forces mostly affect the SMC, since it has lower mass, and is less gravitationally bound, and the ram pressure stripping mostly affects the LMC, because it has a larger reservoir of gas. In physics, ram pressure is pressure exerted on a body which is moving at supersonic velocity through a fluid medium. ...


See also

The Large Magellanic Cloud (also known as LMC) is a dwarf galaxy that is in orbit around our own Milky Way galaxy. ... The Small Magellanic Cloud (SMC) —also known as NGC 292— is an irregular galaxy, sometimes classified as a dwarf galaxy, in orbit around the Milky Way Galaxy. ... The two Magellanic Clouds are irregular dwarf galaxies orbiting our Milky Way galaxy, and thus are members of our Local Group of galaxies. ... In physics, ram pressure is pressure exerted on a body which is moving at supersonic velocity through a fluid medium. ... The Parkes Observatory in New South Wales, Australia The Parkes Observatory is a radio telescope observatory, 20 kilometres north of the town of Parkes, New South Wales, Australia. ... A light year, abbreviated ly, is the distance light travels in one year: roughly 9. ... The parsec (symbol pc) is a unit of length used in astronomy. ...

References

discovery: Wannier, P.; Wrixon, G. T. (1972). An Unusual High-Velocity Hydrogen Feature. ApJ 173: L119 – L123.


MC connection made: Mathewson, D. S.; Cleary, M. N.; Murray, J. D. (1974). The Magellanic stream. ApJ 190: 291 – 296.


initial modelling: Murai, T.; Fujimoto, M. (1980). The Magellanic Stream and the Galaxy with a Massive Halo. PASJ 32: 581 – 604.


LAF discovery: Putman, M. E et al. (1998). Tidal disruption of the Magellanic Clouds by the Milky Way.. Nature 394: 752.


Latest models:


Yoshizawa, Akira M.; Noguchi, Masafumi (2003). The dynamical evolution and star formation history of the Small Magellanic Cloud: effects of interactions with the Galaxy and the Large Magellanic Cloud 339: 1135 – 1154.


Mastropietro, C.; Moore, B.; Mayer, L.; Wadsley, J.; Stadel, J. (2005). The gravitational and hydrodynamical interaction between the Large Magellanic Cloud and the Galaxy. MNRAS 363: 509 – 520.


Connors, Tim W.; Kawata, Daisuke; Gibson, Brad K. (2005). "N-body simulations of the Magellanic Stream"


 
 

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