Sagittarius Dwarf Elliptical Galaxy
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The Sagittarius Dwarf Elliptical Galaxy (Sag DEG) is a satellite galaxy of the Milky Way Galaxy. Roughly 10,000 light years in diameter, it is currently about 70,000 light years from Earth and travelling in a polar orbit at a distance of about 50,000 light years from the core of the Milky Way (about a third the distance of the Large Magellanic Cloud).
Although it is one of the closest companion galaxies to the Milky Way, it is on the opposite side of the galactic core from Earth, and consequently is very faint, although it covers a large area of the sky. It was only discovered in 1994, by R. Ibata, M. Irwin, and G. Gilmore; was immediately recognized as being the nearest known neighbour to our Milky Way. Sag DEG appears to be an older galaxy, with little interstellar dust and composed largely of Population II stars, older and metal-poor compared to the Milky Way.
Based on its current trajectory, Sag DEG is poised to pass through the galactic disc of the Milky Way within the next hundred million years, and is in the process of slowly being absorbed into the larger galaxy.
The exact history and future of Sag DEG is very much under debate.
At first, many astronomers thought that the SagDEG had already reached an advanced state of destruction, so that a large part of its original matter was already mixed with that of the Milky Way. However, Sag DEG still has coherence as a dispersed elongated ellipse, and appears to move in a roughly polar orbit around the Milky Way as close as 50,000 light years from the galactic core. Although it may have begun as a ball of stars before falling towards the Milky Way, SagDEG is now being torn apart by immense tidal forces over hundreds of millions of years. Numerical simulations suggest that stars ripped out from the dwarf would be spread out in long streamers along its path, which were subsequently detected.
However, some astronomers contend that Sag DEG has been in orbit around the Milky Way for some billions of years, and has already orbited around it around ten times. Its ability to retain some coherence despite such strains would indicate an unusually high concentration of dark matter within that galaxy.
On the other hand, others note the similarity of star types between Sag DEG and the Large Magellanic Cloud, arguing that it has recently gone into orbit about our galaxy having somehow become detached.
SagDEG has a globular cluster, M54.
From its discovery until 2003, it was considered to be the closest outside galaxy to Earth, but since then has been overtaken by the newly discovered Canis Major Dwarf Galaxy.
Sag DEG should not be confused with SagDIG, the Sagittarius Dwarf Irregular Galaxy, a small galaxy nearly 4 million light years distant.
See also
External links
- SagDEG (http://www.seds.org/messier/more/sagdeg.html) (SEDS)
- SagDEG (http://www.solstation.com/x-objects/sag-deg.htm) at solstation.comde:Sagittarius-Zwerggalaxie
fr:Sagittaire (galaxie naine elliptique) it:Galassia Nana Ellittica del Sagittario