Magellanic Stream

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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 was discovered as a Neutral Hydrogen (HI) gas feature near the Magellanic Clouds by Wannier & Wrixon in 1972, and the connection to the Magellanic Clouds was made by Mathewson et al. in 1974. Previous to that, in 1965, 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.

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[edit] Observations

Owing to the closeness of the Magellanic Clouds, 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.

[edit] Models

Models had been produced for the formation of the Magellanic Stream 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 Magellanic Clouds (they were tidal models, and just like tides on 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 Magellanic Clouds were actually fully connected to the Magellanic Clouds, and 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.

New models since then have all had to make sure this Leading Arm Feature 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 Small Magellanic Cloud, since it has lower mass, and is less gravitationally bound, and the ram pressure stripping mostly affects the Large Magellanic Cloud, because it has a larger reservoir of gas.

[edit] See also

[edit] 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". MNRAS 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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