A bridge of stars between the Magellanic Clouds

The Magellanic Clouds (Image V Belokurov, D Erkal, A Mellinger)
The Magellanic Clouds (Image V Belokurov, D Erkal, A Mellinger)

An article published in the journal “Monthly Notices of the Royal Astronomical Society” describes the evidence gathered of the existence of a bridge of stars between the two Magellanic Clouds, the two dwarf galaxies satellite of the Milky Way. An international team led by astronomers from the University of Cambridge used data collected by ESA’s Gaia space probe to determine that the bridge is composed not only of gas but also of stars that are old and were stripped from their galaxies.

The Magellanic Bridge was discovered in 1963 but for a long time astronomers thought that there was almost only gas, mostly hydrogen, to connect the Large Magellanic Cloud and the Small Magellanic Cloud, with few stars. In the ’80s examples of star formation were identified so there were very young stars in this gas but a new research could revolutionize our knowledge of that area.

The Gaia space probe, launched in December 2013 to catalog stars not only in the Milky Way, allowed to gather information about the stars in the Magellanic Bridge and to understand their characteristics. The results were compared with data collected by other investigations such as Galex in the ultraviolets and another conducted with the Australian Parkes telescope.

The result is that the Magellanic Bridge seems actually formed by a number of different bridges that don’t coincide perfectly. A thin bridge of very ancient stars classified as RR Lyrae variables, stars that are moving towards the last phase of their lives and show periodic pulsations, turned out to be of particular interest. Those stars have been there since the Magellanic Clouds’ birth so they can provide information on those two dwarf galaxies.

Studying the bridge formed by RR Lyrae variable helps better understand the interactions between the Magellanic Clouds and those between the two dwarf galaxies and the Milky Way. In particular, astronomers are interested in stellar streams that are caused by those interactions and so far had only been a hypothesis. According to the authors of this research, these stars have been in part stripped from the Small Magellanic Cloud by the Great Cloud but another part may have been stripped by the Milky Way from the Large Magellanic Cloud.

Computer simulations were performed and compared with the data collected by the Gaia space probe. The results indicate that many of the RR Lyrae variables that form the bridge were stripped from the Small Magellanic Cloud about 200 million years ago, the last time the two clouds passed relatively close to each other. On that occasion, it’s possible that the hydrogen was snatched by Small Cloud forming the bridge initially discovered.

Probably there are bridges of that kind in many other galaxy groups but it’s difficult to study them millions if not billions of light years away. The relative closeness of the Magellanic Clouds makes them easier to study, also in relation to the amount of the galactic halo gas surrounding galaxies like the Milky Way. The interactions with formations such as the Magellanic Bridge allow to better estimate the amount of matter existing in these galaxies and improve our models of their formation and evolution.

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