Galactic Cannibalism: Our Milky Way Galaxy Has The Munchies

This beautiful, eerie silhouette of dark dust clouds against the glowing nucleus of the elliptical galaxy NGC 1316 may represent the aftermath of a 100 million year old cosmic collision between the elliptical and a smaller companion galaxy.

It appears our Milky Way galaxy has a large appetite, galactic cannibalism is the term given to the process by which a large galaxy, through tidal gravitational interactions with a companion galaxy, merges with that companion, resulting in a larger galaxy. A technical way of saying the big fish eats the little fish.

The current thinking on the formation of galaxies implies that dwarf galaxies were the first to form in the universe. Many of these first dwarf galaxies either went on to clump together to form larger galaxies, or were eventually gobbled up by larger galaxies that continued to grow in this fashion.

Solid observational proof of galactic cannibalism was just on the horizon in 1994 when a new dwarf galaxy, Sagittarius, was discovered very close to the Milky Way and located on the diametrically opposite side of the galactic center from the sun.

It had been suspected for some time that the Milky Way had grown to it’s current size by devouring smaller galaxies, with the discovery of Sagittarius some observational evidence supporting this notion had been found. Now it would be possible to directly view the destruction of a dwarf galaxy as it is being engulfed by the Milky Way galaxy. All that was needed to be done was to find stars that had originally formed part of the dwarf galaxy, but would now be strewn along its entire orbit around the Milky Way. This would constitute two streams encircling the Milky Way. The only problem with this is that the streams would be extremely diffuse and may therefore be completely indistinguishable.

In 1998, investigators from the University of Michigan found the remains of one of the streams that extended to the southwest. These remnants are the furthest from the center of a progenitor galaxy ever detected and confirm that the Sagittarius galaxy has formed an arc that completely surrounds our galaxy, just as predicted by theoretical models. This discovery proves not only that Sagittarius is in an advanced stage of destruction but also - more importantly that the process we call cannibalism has played and continues to play an important role in the formation of the Milky Way.