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Tuesday, December 1, 2009

Sagittarius DEG

Sagittarius DEG -- Dwarf Ellptical Galaxy -- is the correct designation of the small galaxy that is merging with the Milky Way, to distinguish it from Sagittarius DIG, Dwarf irregular Galaxy.

Astronomers have long suspected that the Milky Way Galaxy was formed from smaller galaxies. Moreover, after becoming a relatively large galaxy, it may have continued to acquire a substantial part of its mass by "devouring" smaller galactic companions that moved too close. Apparently confirming that hypothesis was the discovery of a new object in 1994 now commonly referred to as the Sagittarius Dwarf Elliptical Galaxy (SagDEG), found very close to the Milky Way on the opposite side of the Galactic Center from the Solar System (Ibata et al, 1995 and 1994). Not to be confused with the Sagittarius Dwarf Irregular Galaxy (SagDIG), SagDEG is the Milky Way's nearest known neighbor and comprised of mostly old, yellowish stars. Astrophysicist Rosemary Wyse of Johns Hopkins University has estimated that as much as 10 percent of the stars in the Milky Way's halo came from dwarf galaxies like SagDEG, merging with the Milky Way over the past eight billion years or so. (In November 2003, astronomers announced that an even closer galaxy (located 25,000 ly from Sol and 42,000 ly from the galactic center) called the Canis Major dwarf may be losing stars to the Milky Way's disk as well.)

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In 1996, a team of astronomers found a stream of stars that were apparently stripped from SagDEG by the Milky Way as a "tidal trail" (Mateo et al, 1996). Extending to the southwest, it can be traced out to 34 degrees from the center of Sagittarius. Although theoretical models predicted the symmetric presence of another stream, extending to the northwest, that could be so long as to completely encircle the Milky Way, this stream was more difficult to find because it would cross the disk of the Milky Way and so be obscured by the dense stars, gas, and dust of the Galactic Center.

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The article goes on to say that recent observations have confirmed the theory that the stream of the incoming stars of Sagittarius DEG actually do encircle the Milky Way.

In 2003, some astronomers modeling SagDEG's movements with a full-sky map of red (M) giant stars attributed to the galaxy that were detected through the Two-Micron All Sky Survey (2MASS) speculated that it was once pulled through the Milky Way's disk very close to Sol's current location (Majewski et al, 2003; and Law et al, 2003).

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