Image Credit: R. Ibata (UBC), R. Wyse (JHU), R. Sword (IoA)
The Sagittarius Dwarf Elliptical Galaxy (Sag DEG, Sgr dE or the Sagittarius Dwarf Spheroidal Galaxy) is a small elliptical loop-shaped satellite galaxy of our Milky Way that lies about 70,000 light-years away from Earth in the constellation of Sagittarius, while it is currently receding from us at approximately 140 kilometers per second. It is roughly 10,000 light-years across and is home to four known globular clusters, including Messier 54.
Sag DEG should not be confused with the Sagittarius Dwarf Irregular Galaxy (Sag DIG), a small galaxy at 3.4 million light-years distant.
SagDEG is one of the most recently discovered members of the Local Group, and is currently in a very close encounter to our Milky Way galaxy. It is apparently in process of being disrupted by tidal gravitational forces of its big massive neighbor in this encounter. It is surprising that the dwarf has not been disrupted for so far. This fact is an indication for the unusually high concentration of dark matter within this small galaxy, which ties the stars stronger to the galaxy by its gravity.
It is a rather old galaxy, with little interstellar dust and composed largely of older and metal-poor stars, even though it has multiple stellar populations, ranging in age from the oldest globular clusters (almost as old as the Universe itself) to smaller populations of stars as young as several hundred million years which are metal-rich.
Sag DEG can be credited with shaping the Milky Way’s spiral arms.
It has orbited the Milky Way, with a period of 550 to 750 million years, about ten times during its billions of years of existence, at a distance of about 50,000 light-years from the galactic core. During these orbits Sag DEG struck our galaxy some 1.9 billion years ago. It then looped over the galactic “north pole” and struck again about 900 million years ago. It is heading back right now, on course for a third clash with the southern face of the Milky Way disk in 10 million years or so.
Telescopic data and detailed simulations show how these galactic collisions have sent streams of stars out in loops in both galaxies. These long streamers continue to swell with stars and are gradually tugged outward by the Milky Way’s rotation into a familiar spiral arm. It’s the dark matter within Sag DEG that provided the initial push.
Sag DEG pays a high price though – sucked inward repeatedly by the Milky Way’s mightier gravity, it’s being ripped apart by the blows, sending huge amounts of its stars and dark matter into the new arms. Its starting mass was about 100 billion times the mass of our Sun, but has already decreased by a factor of two or three.
When all that dark matter first smacked into the Milky Way, 80 percent to 90 percent of it was stripped off. That first impact triggered instabilities that were amplified, and quickly formed the spiral arms of our galaxy.
Impacts between galaxies and their companions are thought to be widespread in the cosmos, and many of the spiral galaxies we can see were probably formed in this way.